Add documentation about sabre recurrent event

Td.md

+graph TD
+subgraph OP
+E[Openpaas frontend] --> |1.PUT| A
+end
+A[Openpaas backend] -->|2.PUT| B
+subgraph Sabre
+B[Sabre] --> |3.Auth| A
+subgraph Plugin
+G[EventRealTime]
+H[IMIPPlugin]
+end
+end
+A --> |4.Auth response|B
+B .->|5.  Create RabbitMQ Message |G
+B .->|5.  Create Mail Message |H
+H -->|7' Trigger the mail | A
+subgraph Mongo
+B --> |6.Write| C(mongoDB)
+end
+subgraph RabbitMQ
+G --> |7.Publish| D(RabbitMQ)
+end
+D-->|7.Publish|A
+A.-> |8.Publish|E
\ No newline at end of file

recurringEvent.md

+# Recurring Event
+
+##*What is it*
+
+A recurring event is a simple ics where a recurring rule has been specified, the rule is written in the ics with the attribute *```RRULE```* 
+
+```
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+SUMMARY:Recurring
+DTSTART:20150227T010000
+DTEND:20150227T020000
+𝗥𝗥𝗨𝗟𝗘:𝗙𝗥𝗘𝗤=𝗗𝗔𝗜𝗟𝗬
+END:VEVENT
+END:VCALENDAR
+```
+
+Here for the example the RRULE say this event is every day.
+
+##*But if i have a exception ?*
+
+*We take a example :*
+
+Here we have a recurring event on 5 days :
+
+|  Event | Event  | Event  | Event  | event  |
+|---|---|---|---|---|
+
+with the ics :
+
+```
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+SUMMARY:Recurring
+DTSTART:20150227T010000
+DTEND:20150227T020000
+𝗙𝗥𝗘𝗤=𝗗𝗔𝗜𝗟𝗬;𝗜𝗡𝗧𝗘𝗥𝗩𝗔𝗟=𝟭;𝗖𝗢𝗨𝗡𝗧=𝟱 // DAILY=EVERY DAY ; INTERVAL: 1 DAY INTERVAL ; 5 TIMES
+END:VEVENT
+END:VCALENDAR
+```
+
+
+Now postpone the event number 3 
+
+| Event | Event |   | Event | event |
+|---|---|---|---|---|
+|   |   | Event |   |   |
+
+the ics gonna have 2 VEVENT one for the recurrence and one for the exception :
+
+```
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+SUMMARY:Recurring
+DTSTART:20150227T010000
+DTEND:20150227T020000
+𝗙𝗥𝗘𝗤=𝗗𝗔𝗜𝗟𝗬;𝗜𝗡𝗧𝗘𝗥𝗩𝗔𝗟=𝟭;𝗖𝗢𝗨𝗡𝗧=𝟱
+END:VEVENT
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+𝗥𝗘𝗖𝗨𝗥𝗥𝗘𝗡𝗖𝗘-𝗜𝗗:𝟮𝟬𝟭𝟱𝟬𝟮𝟮𝟴𝗧𝟬𝟭𝟬𝟬𝟬𝟬
+SUMMARY:Recurring
+DTSTART:20150228T030000
+DTEND:20150228T040000
+END:VEVENT
+END:VCALENDAR
+```
+
+As you can see in the second VEVENT we have the *RECURRENCE-ID* who determine the exception, the *RECURRENCE-ID* have a date format (YYYYMMDDTHHmmss) where the value is the date of the event exception.
+
+##*How it work with sabre*
+
+Event creation step :
+
+![Schema](./schema.png)
+
+![Schema](./sequence.png)
+
+1. Openpaas send a PUT request to the backend with the event on jcal format.
+
+2. DAV proxy (`linagora.esn.dav.proxy`) is used as a proxy (thanks captain) to transform some of the request to specific method (REPORT, PROPFIND).
+
+Example of body :
+```json
+["vcalendar",[],[["vevent",[["uid",{},"text","b1ad4904-dcef-4a19-9259-29b26f65ac56"],["transp",{},"text","OPAQUE"],["dtstart",{"tzid":"Europe/Berlin"},"date-time","2019-02-19T17:30:00"],["dtend",{"tzid":"Europe/Berlin"},"date-time","2019-02-19T18:00:00"],["class",{},"text","PUBLIC"],["summary",{},"text","EVENT"],["organizer",{"cn":"toto tata"},"cal-address","mailto:toto@linagora.com"],["attendee",{"partstat":"ACCEPTED","rsvp":"FALSE","role":"CHAIR","cutype":"INDIVIDUAL"},"cal-address","mailto:toto@linagora.com"]],[]],["vtimezone",[["tzid",{},"text","Europe/Berlin"]],[["daylight",[["tzoffsetfrom",{},"utc-offset","+01:00"],["tzoffsetto",{},"utc-offset","+02:00"],["tzname",{},"text","CEST"],["dtstart",{},"date-time","1970-03-29T02:00:00"],["rrule",{},"recur",{"freq":"YEARLY","bymonth":3,"byday":"-1SU"}]],[]],["standard",[["tzoffsetfrom",{},"utc-offset","+02:00"],["tzoffsetto",{},"utc-offset","+01:00"],["tzname",{},"text","CET"],["dtstart",{},"date-time","1970-10-25T03:00:00"],["rrule",{},"recur",{"freq":"YEARLY","bymonth":10,"byday":"-1SU"}]],[]]]]]]
+```
+
+3. Sabre send a auth request to Openpaas on the `/api/login` to check if the user have the right  *(cf. DAV/Auth/Backend/Esn.php::validateUserPass)*.
+
+4. Openpaas respond with a token
+
+5. Sabre generate the message to be published on rabbit before the write into Mongo,
+   
+6. Create/Update the event into mongoDB
+
+7. Publish the message into rabbit
+
+8. Via Socket.io (websocket) publish the message to the frontend to apply the creation or modification for the organizer and the attendee (if it's a openpaas user)
+
+
+PS: it's a very simplified schema.
+
+
+##The Difference
+
+Sabre handle a recurring event like a simple event, But diverge when a exception is created. 
+For the example we re-take the ics : 
+
+```
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+SUMMARY:Recurring
+DTSTART:20150227T010000
+DTEND:20150227T020000
+𝗙𝗥𝗘𝗤=𝗗𝗔𝗜𝗟𝗬;𝗜𝗡𝗧𝗘𝗥𝗩𝗔𝗟=𝟭;𝗖𝗢𝗨𝗡𝗧=𝟱
+END:VEVENT
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+𝗥𝗘𝗖𝗨𝗥𝗥𝗘𝗡𝗖𝗘-𝗜𝗗:𝟮𝟬𝟭𝟱𝟬𝟮𝟮𝟴𝗧𝟬𝟭𝟬𝟬𝟬𝟬
+SUMMARY:Recurring
+DTSTART:20150228T030000
+DTEND:20150228T040000
+END:VEVENT
+END:VCALENDAR
+```
+
+When the organizer modify one of this recurrent event, it appear like that on the Database, but if we add a attendee in the exception
+
+Organizer :
+
+```
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+SUMMARY:Recurring
+DTSTART:20150227T010000
+DTEND:20150227T020000
+𝗙𝗥𝗘𝗤=𝗗𝗔𝗜𝗟𝗬;𝗜𝗡𝗧𝗘𝗥𝗩𝗔𝗟=𝟭;𝗖𝗢𝗨𝗡𝗧=𝟱
+END:VEVENT
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+𝗥𝗘𝗖𝗨𝗥𝗥𝗘𝗡𝗖𝗘-𝗜𝗗:𝟮𝟬𝟭𝟱𝟬𝟮𝟮𝟴𝗧𝟬𝟭𝟬𝟬𝟬𝟬
+ATTENDEE;CN=Two:mailto:two@example.org
+SUMMARY:Recurring
+DTSTART:20150228T030000
+DTEND:20150228T040000
+END:VEVENT
+END:VCALENDAR
+```
+
+here we just add a attendee in the exception, but now from the attendee :
+
+```
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:75EE3C60-34AC-4A97-953D-56CC004D6705
+𝗥𝗘𝗖𝗨𝗥𝗥𝗘𝗡𝗖𝗘-𝗜𝗗:𝟮𝟬𝟭𝟱𝟬𝟮𝟮𝟴𝗧𝟬𝟭𝟬𝟬𝟬𝟬
+ATTENDEE;CN=Two:mailto:two@example.org
+SUMMARY:Recurring
+DTSTART:20150228T030000
+DTEND:20150228T040000
+END:VEVENT
+END:VCALENDAR
+```
+
+The attendee have a Event with the *RECURRENCE-ID* and it linked to the 'master event' with the *UID*.

sequence.md

+sequenceDiagram
+    OP Frontend->>OP Backend: PUT
+    OP Backend->>Sabre: PUT
+    Sabre->>OP Backend: Auth api/login
+    OP Backend->>Sabre: Auth response
+    Sabre-->>Sabre: Create RabbitMQ message
+    Sabre-->>Sabre: Create Mail message
+    Sabre->>MongoDB: Create/Update
+    Sabre-->>RabbitMQ: Publish message
+    RabbitMQ-->>OP Backend: Publish message
+    OP Backend-->>OP Frontend: Publish message via WS
+    Sabre->>OP Backend: POST /calendars/inviteattendees
\ No newline at end of file