Update installation instructions for Coq 8.12 and recent versions of FCSL-PCM

For coq-fcsl-pcm package release v1.3 breaks compatibility with the lower 1.x versions, but there is a workaround:
```
opam install coq-fcsl-pcm.1.2.0
```

For coq-htt (the master version builds fine with Coq 8.12, but the opam file does not allow it):
```
opam install coq-htt --ignore-constraints-on=coq
```