Add link from docs to github

[UnePierre]

I think the docs on https://bumper.readthedocs.io/ could profit from a prominent link to this project on GitHub.
Maybe docs/index.md would be the right place to mention, where the source can be found?

[UnePierre]

Hey, I found one link to GitHub on docs/origins.md pointing to the old v0.1.0 version.

Another link to the project would go into docs/Develop.md, IMO.