Install the release notes in configure's docdir

[dbuenzli]

Finding the release notes of the compiler you are using in your opam switch is always a very frustrating and time consuming experience.

It could be much easier if Changes was installed in the configure's docdir directory. It shouldn't then be hard for the opam ocaml package to set that directory to its opam var doc directory convention (which does not follow the standard configure layout, which has the docdir directory in share).

That way any of these exceedly simple invocations will do the job:

less $(opam var doc)/ocaml/Changes
less $(opam var ocaml:doc)/Changes 
odig changes ocaml

There's certainly other things that could be installed there like the README, but I care a bit less about this one.

[nojb]

This sounds reasonable. Would you like to propose a PR? It should just be a matter of modifying the install target in the main Makefile.

[dbuenzli]

I'm afraid not. The reason why I didn't open a PR is that I think its more complicated than that :-) I'm not longer really familiar with the build system and autoconf machinery.