Update information about supported OSs

setup-and-getting-started/README.md

@@ -6,22 +6,24 @@
 
 Two builds with the key4hep stack are distributed on cvmfs. The releases happen
 every few months on demand (for example, if there is a new important feature or
-a breaking change) and at the moment only CentOS 7 is supported (support for
-AlmaLinux 9 and Ubuntu is coming soon). Run the following to set up the stack:
+a breaking change) and at the moment Ubuntu22.04 and AlmaLinux9 (EL9,
+RockyLinux9) are supported. We also have older releases for CentOS7 but are not
+making any new builds for that.
 
 ```bash
 source /cvmfs/sw.hsf.org/key4hep/setup.sh
 ```
 
-In addition, nightly builds for CentOS 7, AlmaLinux 9 and Ubuntu 22.04 with the
-latest version of most of the packages are available:
+In addition, nightly builds for AlmaLinux 9 and Ubuntu 22.04 with the latest
+version of most of the packages are available:
 
 ```bash
 source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh
 ```
 
 The `setup.sh` script always points to the latest build and it will change
 without warning. However, after sourcing the script some information will be
-displayed with instructions on how to reproduce the current environment. Nightly
-builds are intended for development and testing and they will be deleted after
-some time from `/cvmfs`.
+displayed with instructions on how to reproduce the current environment.
+**Nightly builds are intended for development and testing and they will be
+deleted after some time from `/cvmfs`. They will also introduce new features
+unannounced, so don't use these for anything else than development!**