Readme and contributing changes

Greg asked volunteers to review these two files ([Discuss] volunteer wanted to clarify README.md and CONTRIBUTING.md) and I did. Feel free to comment.

After squashing (and accidentally squashing too much and splitting again) I put the stuff into a new branch. At least, rebuilding history with Git works well. First pull request was #537.

Feedback on how I should have done this instead are welcome ;D
"in the Makefile as well." -> "in the `Makefile` as well."