Added .md extension to INSTALL files

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10083 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
Updated INSTALL file references in the Xcode project

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10084 7b3f36aa-e6db-49c1-8b4c-66eac1997e64