add CONTRIBUTING file

[skip ci]
Update CONTRIBUTING.md
add CLA
ocamlbuild deletes the link, so use the binary directly
make an ocaml pack for libmerr
build merr twice, once for bootstrapping
use ppa to install custom menhir
Merge branch 'master' of github.com:pippijn/merr
don't wait for user input to install ppa
reload packages after adding ppa
update link to contributing page
Move libmerr.mlpack to root to fix the build.
Refactor _tags.