Build instructions fail

[nikos-alexandris]

Currently both ways of building from source listed in INSTALL.md (using opam and using Makefiles) fail on a fresh clone of master due to dependency resolution errors. Suggest the build instructions (at least for the opam version) get replaced with just

git clone https://github.com/CertiCoq/certicoq
cd certicoq
opam switch create --repos=default,coq-released .

This creates a sandboxed environment without explicit pins and no unmentioned dependencies. For better sandboxing and reproducibility a Dockerfile could also be helpful.