Add branch description to project README

README.md

@@ -3,12 +3,24 @@ ATLAS Offline software
 
 Welcome to [ATLAS](https://atlas.ch) offline software.
 
-If you fork this project, please don't forget to add the ATLAS Robot as a developer (assuming you might want to submit a merge request) as documented [here](https://atlassoftwaredocs.web.cern.ch/gittutorial/gitlab-fork/).
+If you fork this project, please do not forget to add the ATLAS Robot as a developer (assuming you might want to submit a merge request) as documented [here](https://atlassoftwaredocs.web.cern.ch/gittutorial/gitlab-fork/).
 
 Please see the [Twiki](https://twiki.cern.ch/twiki/bin/view/AtlasComputing/AtlasComputing) for more information, or look below for some other links.
 
+Branches
+--------
+The following branches are currently active:
+
+Branch                                                            | Purpose                      | Main Project
+------------------------------------------------------------------| ---------------------------- | ---------------------------------------
+[21.0](https://gitlab.cern.ch/atlas/athena/tree/21.0)             | Tier0 and MC16c              | Athena,21.0.X
+[21.0-mc16a](https://gitlab.cern.ch/atlas/athena/tree/21.0-mc16a) | MC16a bugfixes               | Athena,21.0.20.Y
+[21.1](https://gitlab.cern.ch/atlas/athena/tree/21.1)             | HLT and P1 monitoring        | AthenaP1,21.1.X
+[21.2](https://gitlab.cern.ch/atlas/athena/tree/21.2)             | Derivations and Analysis     | AthDerivations, (Ath)AnalysisBase, 21.2.X
+[master](https://gitlab.cern.ch/atlas/athena/tree/master)         | AthenaMT development         | Athena,22.0.X
+
 Links
-------------------------
+-----
 
 - The [ATLAS webpage](https://atlas.ch) will tell you all about the ATLAS experiment (for ATLAS members, the collaboration webpage is [here](http://atlas.web.cern.ch/Atlas/Collaboration/))
 - The [Twiki](https://twiki.cern.ch/twiki/bin/view/AtlasComputing/AtlasComputing) is the main source of documentation, and has many links to sub-domains