document issue and pull request guidelines in contributing doc

[techieshark]

To make it easier for others to contribute to the project, we should document what we're looking for in issue descriptions and pull requests.

If we create a CONTRIBUTING.md file, Github will include a handy link to it from the new issue page and new pull request page: https://github.com/blog/1184-contributing-guidelines

Here's a good example: https://github.com/thoughtbot/factory_girl_rails/blob/master/CONTRIBUTING.md

A few things we might mention:

• code style guidelines
• commit message guidelines (like this)
• tips for keeping branches simple (see part 3 of this comment)

Later we could set up a lint bot to check code style etc but for now just being clear about these guidelines should help.

[Lilith-Palmer]

@Rustuma have you thought about this?

[techieshark]

I'll take a stab at this.