Write up info about getting started with contributing

[alexgleason]

I propose this:

1. We first create a document that gives a better high-level explanation of how GardenHub works, perhaps even with videos.

2. Walk users through setting up their dev environment (this is already covered in the readme, but we could make this easier and solve the current Docker bugs...)

3. Put more effort into creating "bitesize" issues that give a lot of detail and information about the problem to be solved, tag them, and direct people to them from the readme and the docs.

In terms of bitesize, I've already tagged #132 and rewritten it in the type of format I'm talking about. Unfortunately it's not as simple as it appears on the surface because Semantic UI doesn't have an obvious way to make an element selected on pageload within a "dropdown," so I plan to add more info to the issue. Either way, you get the point.