Add more documentation about crates.io's database needs

[carols10cents]

Fixes #541.

[Susurrus]

@alexcrichton Quick on the draw there, I was actually just reviewing these notes by setting up a test environment on my own computer. I found them a tad confusing if you're looking to just run the tests without worrying about self-hosting a complete backend. For me, just wanting to run the tests, I skipped over the "Working on the Backend" section to get to the "Running Tests" section, but had to backtrack because steps 1 & 2 are necessary (but none of the others are) to run the tests.

Therefore I would suggest splitting the section into 3 parts:

1. First specify that postgres needs to be installed and the backend needs a .env file.
2. Put steps 3+ from the original "Working on the Backend" section into a "Self-hosting with the Backend" title or something.
3. Adjust the "Running Tests" section to account for Steps 1 & 2 above.

[carols10cents]

@Susurrus now that the information is there and could just use some rearranging, do you have time to send in a PR for that? I'm pretty busy today :)

[Susurrus]

Sure! I'll do that but it might be tomorrow.