reworking CONTRIBUTING to be more than bullet points

[Rendered Markdown - CONTRIBUTING](https://github.com/octokit/octokit.net/blob/cleanup-laughing-octo-sansas/CONTRIBUTING.md)
[Rendered Markdown - README](https://github.com/octokit/octokit.net/blob/cleanup-laughing-octo-sansas/README.md)
[Rendered Markdown - DEPLOYMENT](https://github.com/octokit/octokit.net/blob/cleanup-laughing-octo-sansas/DEPLOYMENT.md)

Any gross abuses of the English language? Or things that could be made clearer?

cc @tbondy760 @nigel-sampson @AndyCross @lbargaoanu @vktr as you are relatively new contributors

 - [ ] coding style guidelines
 - [x] a bit more git-fu