Improve CONTRIBUTE and related files.

* CONTRIBUTE: improve; add explicit web references, move some info from
  admin/notes/* here.

* INSTALL.REPO: You can't "just run make" after a clean checkout.

* admin/notes/commits: deleted; merged into ./CONTRIBUTE

* admin/notes/repo: move commit, branch info into ./CONTRIBUTE
* CONTRIBUTE: minor improvements

* ChangeLog: cleanup entries for my recent commits