Update 18f-source-code-policy.md

Here's a bit of tinkering. Comments are in markdown. 

Following Eric's suggestion, I tried to make this a more human-oriented. I think this can serve as an internal "north star" as we think about how we expand this line of thinking to the rest of the agency. Welcome any and all suggestions for how to modify.
A couple typo fixes, formatting fix for bullets, and a proposed end of sentence
Merge pull request #1 from konklone/patch-2

A couple typo fixes, formatting fix for bullets...
Update 18f-source-code-policy.md
Merge pull request #2 from rjmajma/patch-1

Update 18f-source-code-policy.md