Add compile time dependency on python to readme

[DKuhse]

Hello,

during the compilation of metacoq-translations using opam install . there is a dependency on python. Since not all distributions ship with python by default, it might be nice to note this somewhere, since the (non-verbose) error is somewhat cryptic:

[ERROR] The compilation of coq-metacoq-translations failed at "/home/daniel/.opam/opam-init/hooks/sandbox.sh build make -j3

using the verbose flag, the relevant output is

/usr/bin/env: 'python3': No such file or directory
make[2]: *** [Makefile.coq:344: print-pretty-timed] Error 127

There also appears to be a dependency on sed, but missing it doesn't appear to cause any issues.

[mattam82]

Thanks for the report! Sadly I don't see an easy way to make the package depend on python in opam