Update INSTALL.md

[NeonOxide]

Update installation instructions to include the hidden dependencies on both Inria's repositories and in clang and libgmp-dev (also removed the pinning of submodules as it breaks the process). Also included the current and arguably easiest of installing coq-certicoq by just using Inria's repositories without even having to clone the repo.

[andrew-appel]

In the section "Dependencies", perhaps it's worth changing "If that is not an option", to something like this:

If you are building a version of CertiCoq that depends on (for example) a bleeding-edge version of MetaCoq that's not yet released into an opam repository, then you may need to base some of your dependencies on a git submodule rather than from opam. You can use these instructions. Note that ...