- Add info about stable and edge.

Thanks to xet7 !

README.md

@@ -1,5 +1,14 @@
 # Wekan
 
+## Stable
+
+- master+devel branch. At release, devel is merged to master.
+- Receives fixes and features that have been tested at edge that they work.
+
+## Edge
+
+- edge branch. All new fixes and features are added to here first. [Testing Edge](https://github.com/wekan/wekan-snap/wiki/Snap-Developer-Docs).
+
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