prepare for Lmod/5.8
improve readme.rst

Signed-off-by: Fotis Georgatos <fotis@cern.ch>
fix deprecated fallback to ConfigureMake easyblock in Lmod 5.8 easyconfig
Merge pull request #1267 from boegel/Lmod_58

add easyconfig for Lmod 5.8 + update accompanying README/sourceme