Add README for data API.

README_DEVELOPER.md

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+# OpenTracks Data API
+
+## Reason
+OpenTracks by itself only provides basic features for data visualization and also lacks functionality for analytics.
+By itself OpenTracks provides a basic dashboard that can show summary of a track (while recording as well as afterwards).
+Although this might be (mostly) sufficient, it has some limitations.
+This include limited configurability (e.g., ordering of shown elements) and the lack of Internet access.
+In fact, the later is considered a feature but severely limits the implementation presenting a map (at least in an easy way). 
+
+_NOTE:_ for showing a track on a map a workaround was implemented in OpenTracks.
+Data could be exported as KMZ and presented in 3rd-party application (e.g., [OsmAnd](https://play.google.com/store/apps/details?id=net.osmand) or [MAPS.ME](https://play.google.com/store/apps/details?id=com.mapswithme.maps.pro)). 
+
+For the time being, the OpenTracks Data API can be used to implement mainly custom dashboards.
+
+## Implementation
+OpenTracks by default does not expose any data to 3rd-party applications.
+On the request of the user (i.e., pressing a button), an Intent is created that exposes the `Track` data (it's summary) as well as the `TrackPoint`s.
+For each, one URI is provided.
+These URIs notify observers if on content changes (e.g., a new `TrackPoint` was added).
+
+Limitations:
+* only tracks selected by the user is exposed,
+* `Waypoints` are not exposed,
+* access is only granted temporarily and automatically revoked (see `FLAG_GRANT_READ_URI_PERMISSION`),
+* no write access is impossible, and
+* neither track recordings can be started, paused, or stopped.
+
+All required information can be found in `IntentDashboardUtils.java`. 
+
+A _reference implementation_ is the [OSMDashboard](https://github.com/OpenTracksApp/OSMDashboard).