Update README about versions

Author: prehensilecode

README.md

@@ -5,6 +5,11 @@ This splits off my pull request https://github.com/deepmind/alphafold/pull/166
 
 Disclaimer: this project is not affiliated with DeepMind.
 
+## IMPORTANT - `main` branch not usable directly
+The `main` branch here is not directly usable with the `main` branch of AlphaFold. This Singularity recipe
+works only with the matching release of AlphaFold. Please use one of the releases here, with a matching
+release of AlphaFold.
+
 ### Unfixed bug in Alphafold tagged release 2.2.2
 N.B. https://github.com/deepmind/alphafold/issues/510#issuecomment-1159062272