CONTRIBUTING: how to sign the CLA
CONTRIBUTING: how to sign the CLA

This summarizes Xavier's recommendations at
  https://github.com/ocaml/ocaml/pull/342#issuecomment-178720630
which were later reused at
  https://github.com/ocaml/ocaml/pull/307#issuecomment-183356057

In particular this should create a URL anchor to direct people to:
  https://github.com/ocaml/ocaml/blob/trunk/CONTRIBUTING.md#how-to-sign-the-cla

gerdstolpmann/ocaml,eliberis/ocaml,msprotz/ocaml,eliberis/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,yunxing/ocaml,msprotz/ocaml,msprotz/ocaml,msprotz/ocaml,eliberis/ocaml,yunxing/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,yunxing/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,eliberis/ocaml,eliberis/ocaml