Added warning in the Git version of the README

Added warning in the Git version of the README

I noticed that some careless hurried people tend to use Monocypher from
the git repository directly. They don't even grab the releases from
GitHub.  That's not ideal for two reasons:

1. The master branch isn't always stable.
2. The Git repository misses some automatically generated files.

This patch attempts to get end users away from the Git repository,
towards well tested official releases.  Also, for users who think the
tarball are binary releases (they're source releases), or just want to
be done as quickly as possible, I also gave direct links to the main
source and header files.