Add some more filetypes as valid GitHub readmes
Add some more filetypes as valid GitHub readmes

(All the markdown file extensions it knows, plus rst and asciidoc. Reference:
 https://github.com/github/linguist/blob/master/lib/linguist/languages.yml )

perl6/modules.perl6.org,perl6/modules.perl6.org,perl6/modules.perl6.org,perl6/modules.perl6.org