Document how to set up a dev environment

[spenczar]

Here's a few quick words on contributing. I'm not attached to how anything is phrased, really.

Related to (but doesn't necessarily fix?) #182.

[codecov]

Codecov Report

Merging #188 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
- Coverage    79.2%   79.15%   -0.06%     
==========================================
  Files          24       24              
  Lines        3804     3804              
==========================================
- Hits         3013     3011       -2     
- Misses        595      596       +1     
- Partials      196      197       +1

Impacted Files	Coverage Δ
source_manager.go	91.38% <0%> (-0.75%)	❌

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[sdboyer]

Good enough for now, I think, especially given that this is going to be moot soon. Thanks!