Proposal: merge content of README.asc and CONTRIBUTING.md

[HonkingGoose]

Which version of the book is affected?

The version that's hosted on GitHub.
Present on master.

Describe the bug:

The documentation for contributing is split between the landing page README.asc and the CONTRIBUTING.md

Steps to reproduce:

1. Go to https://github.com/progit/progit2.
2. A contributor reads the section that displays the README.asc
3. But then also the contributor needs to click trough to another guide, to find out how to really contribute.

Expected behavior:

1. Go to https://github.com/progit/progit2
2. The README.asc contains everything a contributor needs to get started, so the content of README.asc and CONTRIBUTING.md are merged.

Screenshots:

Additional context:

I noticed the split between the files when working on the text of the contributing guide in this PR: #1477.

Discussion:

What is the reason that the content is spread around in two files, instead of merged into one README?
If you want to merge the content, I'll make a pull-request that merges the content together.

[ben]

Probably the biggest reason is this feature of GitHub:

This shows up even more prominently when you're opening a new PR. I know that the audience for both documents is pretty similar, but this one feature means that having a specific document for contribution guidelines makes a lot of sense.

[HonkingGoose]

Ah okay, I'll close this issue in favor of making a PR template, as I suggested, and you wanted in #1479

Thanks for merging in my work on a lot of pull-requests! 😄