Documenting mock-scm plugin

[Miroslav Suchý]

FIY:
I wrote documentation for mock-scm
  https://fedoraproject.org/wiki/Projects/Mock/Plugin/Scm

I find some bugs during writing this doc and it will be fixed in next version of mock.

If you want to enhance this doc, feel free to do it.
-- 
Miroslav Suchy, RHCA
Red Hat, Senior Software Engineer, #brno, #devexp, #fedora-buildsys