Add initial app-cdr/isomd5sum ebuild

Still needs testing
add some more info
more links and notes
moved note to ./README.asciidoc
add more options to layout.conf
add example on how to mask whole overlay
Merge branch 'master' of https://github.com/aucampia/aucampia-gentoo-overlay
Merge branch 'master' of https://github.com/aucampia/aucampia-gentoo-overlay