Add more info to README

README.md

@@ -1,7 +1,24 @@
 # paraview-build
-A repository with a GitHub action to build and package 
+A repository with a GitHub action to build and package[^1]
 [ParaView](https://github.com/Kitware/ParaView) using the
-[Superbuild](https://gitlab.kitware.com/paraview/paraview-superbuild)
-with the following options:
+[Superbuild](https://gitlab.kitware.com/paraview/paraview-superbuild) on the
+Ubuntu 20.04 GitHub action runners.
+
+The build is/will be used in the GitHub action which tests the [ParaView
+Catalyst](https://www.paraview.org/in-situ/) instrumentation in GRChombo. It is
+built against the Ubuntu 20.04 system libraries so is unlikely to work on other
+Linux distributions with different library/package versions.
+
+The build has the following features:
 * Catalyst Rendering edition
 * OSMesa rendering
+* Python 3 support
+* MPI
+* LLVM support
+* Release build with debugging info
+
+[^1]: Note that as the Superbuild `cpack` targets do not currently include the
+paraview-config script which is required for building GRChombo with ParaView
+Catalyst in-situ support, the packaging in this action is rudimentary and just
+makes a tarball from the relevant build directory. This is another reason why
+the builds in this repository are even less likely to be portable.