Add CONTRIBUTING.md

[graceyim]

Add first draft of CONTRIBUTING.md.

[coveralls]

Coverage remained the same when pulling c343402 on add-contributing-readme into dc70c64 on develop.

[smholloway]

Would it be helpful to show what a successful test run would look like? For example,

Upon completion, you should see something like this:

....................................
----------------------------------------------------------------------
Ran 36 tests in 0.016s

OK

[coveralls]

Coverage remained the same when pulling 54cc1cb on add-contributing-readme into dc70c64 on develop.

[asteinwedel]

Instructions worked for me.