include safeguards in `master` branch

[jsign]

We highly recommend users avoid running master code, but specific release tags/commit.

We tried some git-flow style of develop as the main development branch, but it has many corner cases that doesn't seem to fit well in Github.

The plan now is to go back to develop on master but:

• Include in the README big warning about master potentially being unstabled and undocumented.
• Encourage always to clone specific tags, not branches.
• Investigate if git-hooks can signal warning messages to notify the user/developer if is in master branch.
• Change the Makefile to warn the user if it's building or running commands in master.