Explaining labels

Explaining what we mean by the various labels that can be put on issues and pull requests.
@gvwilson I know `wont-fix` is used frequently but it is unwelcoming to new contributors. It would be better to use a word that is clear that no further action will be taken now on this issue, yet doesn't have the heavy handed response that wont-fix has. Perhaps something more along the lines of outside of project scope (too long for a label) or tabled or shelved. I will see if I can propose some other suggestions that are used in practice.