Doc: CG: clarify convert-ly usage policy. (issue2687043)

[percival . music . ca]

Reviewers: ,

Message:
Policy highlights:
- convert-ly will be the responsibility of the programmer, not the
doc+translation people.  (however, translators are still on the hook for
the current round of updates, as per issue 1358)
- easy copy&paste lines to run convert-ly


Description:
Doc: CG: clarify convert-ly usage policy.

Please review this at http://codereview.appspot.com/2687043/

Affected files:
  M Documentation/contributor/doc-work.itexi
  M Documentation/contributor/programming-work.itexi