Update project README

Update project README

Update the git project README file with updated links and branch information.

README.md

 ATLAS Offline software
 ========================
 
-Welcome to [ATLAS](https://atlas.ch) offline software!
+Welcome to [ATLAS](https://atlas.ch) Athena software!
 
-To find out how to build Athena, make merge requests etc, please have a look at the [release building](https://atlassoftwaredocs.web.cern.ch/guides/build_release/) pages.
-
-In particular, 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/).
+To contribute to this project, follow our [git tutorial](https://atlassoftwaredocs.web.cern.ch/gittutorial/). In particular, if you fork this project *please do not forget* to [add the ATLAS Robot as a developer](https://atlassoftwaredocs.web.cern.ch/gittutorial/gitlab-fork/).
 
 For ATLAS members, please see the [Atlas Computing Twiki](https://twiki.cern.ch/twiki/bin/view/AtlasComputing/AtlasComputing) for even more information.
 
 Branches
 --------
-The following branches are currently active:
+The following major branches are currently active:
 
 Branch                                                            | Purpose                      | Main Project, Release Series
 ------------------------------------------------------------------| ---------------------------- | ---------------------------------------
 [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
 [21.0-mc16d](https://gitlab.cern.ch/atlas/athena/tree/21.0-mc16d) | MC16d 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
-[21.3](https://gitlab.cern.ch/atlas/athena/tree/21.3)             | MC18 Simulation              | Athena, 21.3.X
 [21.6](https://gitlab.cern.ch/atlas/athena/tree/21.6)             | Event Generation             | AthGeneration, 21.6.X
-[21.9](https://gitlab.cern.ch/atlas/athena/tree/21.9)             | Upgrade Phase-2 developments | Athena + AthSimulation, 21.9.X
-[master](https://gitlab.cern.ch/atlas/athena/tree/master)         | AthenaMT development         | Athena, 22.0.X
+[22.0](https://gitlab.cern.ch/atlas/athena/tree/22.0)             | Run-3 Tier0 and Point1       | Athena, 22.0.X
+[master](https://gitlab.cern.ch/atlas/athena/tree/master)         | Development, Analysis, Derivations | all projects
 
 Links
 -----
 
 - The [ATLAS webpage](https://atlas.ch) will tell you all about the ATLAS experiment (for ATLAS members, the collaboration webpage is [here](https://atlas-collaboration.web.cern.ch/))
 - The [Twiki](https://twiki.cern.ch/twiki/bin/view/AtlasComputing/AtlasComputing) is the main source of documentation, and has many links to sub-domains
-- The [software documentation](https://atlassoftwaredocs.web.cern.ch) page has some tutorials etc
+- The [software documentation](https://atlassoftwaredocs.web.cern.ch) page is the main entry point for tutorials, etc.
 - The git repository is [here](https://gitlab.cern.ch/atlas/athena)
-- And the Doxygen code documentation can be found [here](https://atlas-sw-doxygen.web.cern.ch/atlas-sw-doxygen/atlas_22.0.X-DOX/docs/html/index.html)
+- The [ATLAS Nightlies and CI page](https://bigpanda.cern.ch/globalview/) shows the build status of all nightlies
+- The [Doxygen code documentation](https://atlas-sw-doxygen.web.cern.ch/atlas-sw-doxygen/atlas_22.0.X-DOX/docs/html/index.html) is available for the master branch
 - To cite Athena, you can use the following zenodo [DOI](https://zenodo.org/record/2641997)