[libre-riscv-dev] renaming master branch

[Jacob Lifshay]

I don't know how important we should deem this, but I thought we
should at least talk about it:

Should we rename the master branch to avoid the association with slavery?
GitHub, Git, and LLVM are all currently discussing this.

If we do decide to change, I'd recommend waiting till Git decides
which name they will use and emulate that to promote consistency.

Jacob Lifshay