Add CONTRIBUTING file to help new contributors be successful

[jaredmorgs]

When working on #32 I wasn't expecting to have to build the app locally for my PR to be accepted.

@fisyher was really helpful in explaining the situation to me, and suggested that I create a CONTRIBUTING.md file as a way of helping new contributors get started quickly.

I will include this work in #121 as a starting point so that other project team members can fill in any gaps I have as a "non-developer" (I'm a Technical Writer that is willing to get my hands dirty a bit).

[limyz]

See https://gist.github.com/PurpleBooth/b24679402957c63ec426 for a sample

[jaredmorgs]

If you haven't done so already @limyz I have made a start on a CONTRIBUTING file in #121 which I will separate from that MR and attach to this one.

Would you be open to having slightly more information in the file compared with the sample linked above?

Or would you prefer the file to mirror the linked example file and then offload any project building instructions into another MD file?