message: Update RELEASE-NOTES about MFEM upgrade
diff: @@ -87,7 +87,8 @@ The Axom project release numbers follow [Semantic Versioning](http://semver.org/
  - Updated built-in TPL `fmt` to master branch snapshot, March 26, 2021.
  - Inlet: SphinxWriter will now print only one element schema per container instead of
  printing the same schema for each element in the container
  -- Update BLT to release 0.4.0
  +- Updated BLT to version 0.4.0 released 9 Apr 2021
  +- Updated MFEM to version 4.2 released 30 Oct 2020. Axom no longer requires MFEM to be built serially
  ### Fixed
  - Updated to new BLT version that does not fail when ClangFormat returns an empty
  