Write a README

[akavel]

• the last paragraph in use-cases.md has the main value quote:

  [...] the main benefit it brings, is that the repo should become fully self-contained, and especially all historic builds (since this solution is introduced) will be reproducible too, with correct versions of dependencies.

• note that the "host" VCS supported is currently only git; list the "guest" VCSes currently supported & mention we're open to others;