Add retrospective and learnings documents

[dmethvin-gov]

Let's be sure we put our information in the repo here and reference it in the readme.

[easherma]

@dmethvin-gov would love to see these docs! we're still rolling with this repo for now at City of Austin, and debating further refactoring vs migrating to a different project.

[dmethvin-gov]

Here's a first draft, I decided to post it in the wiki so we can edit it when needed. I've also attached a presentation in PowerPoint format that covers most of the same points.
USFS_Retrospective_Final.pptx

https://github.com/usds/us-forms-system/wiki/Lessons-from-the-Post-Phase-1-Discovery-Sprint

[easherma]

Awesome, thanks so much!