Complete some of the pending changing in renaming some files to .md
Created by: JosephKiranBabu
In commit d4b47917, four files were renamed by adding a file extension .md The files are CONTRIBUTING, COPYING, HACKING and INSTALL This commit fixes the pending tasks and especially the bug where HACKING.md is wrongly listed as HACKING in debian/plinth.docs and cause build failure for the Debian package of Plinth
Signed-off-by: Joseph Nuthalapati njoseph@thoughtworks.com