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
I know that the previous items use `<br/>` but AFAIK Markdown understand the indentation as list continuation. So, could we use

~~~
* First Item

  Continuation
* Second item   
~~~