Clarify readme and contributing

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.
> What are this "certain conditions"?

This is explained in the next sentence. I thought about writing "under certain conditions only (see below)", but then I realized that the explanation already starts in the next sentence. And, because the explanation starts in the next sentence, I decided that a "see below" would confuse people more than not telling them where to find these rules.