Add PULL_REQUEST_TEMPLATE.md & CONTRIBUTING.md files

[colin-marshall]

@Aetles made some great points regarding documentation of commits in #1123 (comment). I think one thing that would help improve this is to add .github/PULL_REQUEST_TEMPLATE.md with some basic info requirements for every pull request. More info:
https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

Along those same lines, we should also add .github/CONTRIBUTING.md to the repo. More info:
https://help.github.com/articles/setting-guidelines-for-repository-contributors/

What do you guys feel we should put in these files?

[olefredrik]

Great initiative! I think this would be useful indeed. As a rule of thumb, I think that all pull requests should be linked to an issue. That way, we want to make sure that the community gets an opportunity to make feedback before potentially unexpected changes suddenly end up in the master branch. This will apply to bug fixes as well as introduction of new features.

(I'm the first to admit that I need to get better on this. As there have been no written guidelines for this, I have not always followed my own best practices.)

Consise and descriptive commit messages should also be written down as part of the guidelines.

Do you agree? Do you have anything specific in mind?

[colin-marshall]

@olefredrik I think an issue corresponding to every pull request is a great idea. That and directions for more informative commit messages will be a great start.