Added API doc

API.md

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+## URL ##
+
+All API endpoints are based off of `/api` if you access Radarr via `http://localhost:7878` the API root would be `http://localhost:7878/api`
+
+
+## Authentication ##
+
+All requests made to the api endpoint require API Key authentication using the X-Api-Key header.
+
+### API Key ###
+
+- Can be accessed and reset via Settings -> General
+- Stored in Config.xml
+- alpha-numeric (lower case)
+
+## Dates & Times ##
+
+- All dates/timestamps are ISO-8601 formatted in UTC `2014-01-27T01:30:00Z`
+- Movies (and missing & calendar) also include the airdate in the original timezone for display purposes
+- Date parameters should be ISO-8601 UTC dates to ensure proper handling by Radarr
+
+## Content Type ##
+
+- All POST/PUT requests require all parameters to be JSON encoded in the body, unless otherwise noted.
+- All GET requests will return a JSON encoded response
+
+## Endpoints ##
+
+- Calendar
+- Command
+- Diskspace
+- History
+- Movies
+- System-Status