README: add format descriptions

README.md

@@ -67,21 +67,47 @@ by default, the program downloads all formats for every activity. Use the
 
 Supported export formats:
 
-  -   ``json_summary``: activity summary file (JSON)
 
-  -   ``json_details``: activity details file (JSON)
+  -   ``gpx``: activity GPX file (XML).
 
-  -   ``gpx``: activity GPX file (XML)
+      [GPX](https://en.wikipedia.org/wiki/GPS_Exchange_Format) is an open
+      format, mainly for storing GPS routes/tracks. It does support extensions
+      and Garmin appears to annotate the GPS data with, for example, heart-rate
+      and cadence, when available on your device.
 
   -   ``tcx``: an activity TCX file (XML).
       *Note: a ``.tcx`` file may not always be possible to export, for example
       if an activity was uploaded in gpx format. In that case, Garmin won't try
       to synthesize a tcx file.*
 
+      [TCX](https://en.wikipedia.org/wiki/Training_Center_XML) (Training Center
+      XML) is Garmin's own XML format. It is, essentially, an extension of GPX
+      which includes more metrics and divides the GPS track into "laps" as
+      recorded by your device (with "lap summaries" for each metric).
+
   -   ``fit``: activity FIT file (binary format).
       *Note: a ``.fit`` file may not always be possible to export, for example
       if an activity was entered manually rather than imported from a Garmin device.*
 
+      The [FIT](https://www.thisisant.com/resources/fit/) format is the "raw
+      data type" stored in your Garmin device and should contain all metrics
+      your device is capable of tracking (GPS, heart rate, cadence, etc). It's a
+      binary format, so tools are needed to read its content.
+
+  -   ``json_summary``: activity summary file (JSON).
+
+      Provides summary data for an activity. Seems to lack a formal schema and
+      should not be counted on as a stable data format (it may change at any
+      time). Only included since it *may* contain additional data that could be
+      useful for developers of analysis tools.
+
+  -   ``json_details``: activity details file (JSON).
+
+      Provides detailed activity data in a JSON format. Seems to lack a formal
+      schema and should not be counted on as a stable data format (it may change
+      at any time). Only included since it *may* contain additional data that
+      could be useful for developers of analysis tools.
+
 All files are written to the same directory (``activities/`` by default).
 Each activity file is prefixed by its upload timestamp and its activity id.