_I recommend reviewing this with whitespace changes disabled._
Signal has renamed its primary branch to `main`. This updates references
to the old name, `master`, to either reference `main` or a specific
commit hash.
I also fixed a couple of small whitespace issues in a file I was
editing.
* Update contributing.md to use signal.org URLs
* Remove Bithub references from readme.md, contributing.md
In hand with:
0bcf46bacd
* Remove Bithub references from PR template