add various PR guidelines to the README

I expect that the main party motivated to actually fix this issue will be the maintainers of this repo, so I intend this issue as mostly a reminder that the maintainers should do so.

There are various guidelines such as:

* If a commit affects one exercise, prefix the commit message with that exercise's name. Do something similar if the commit affects one clear area of the repository.
* If the PR follows a problem-specifications change, link to the relevant PRs from problem-specifications.
* If amending commits while PR is in review, it's helpful to include a verbal explanation of what changed. No preference between amending versus just adding new commits.
* Mention https://help.github.com/articles/closing-issues-using-keywords/
* Each commit should make a single logical change. Maintainers do have the ability to squash every commit in a PR into one commit when merging, and we use that ability when it is appropriate. If the PR should instead be multiple commits, maintainers might choose to manually arrange the changes into the appropriate commits or ask the submitter to do so. Submitters should feel free to speak up about any preferences or suggestions as to whether the final product should be one commit or multiple.

Should add those in some appropriate location.