docs: add dev note about feature toggles

[nsprenkle]

TL;DR - I wanted a note somewhere about where ORA-specific feature toggles live. I added a brief note.

JIRA: https://openedx.atlassian.net/browse/AU-284

Developer Checklist

• [x] Reviewed the release process
• n/a Translations and JS/SASS compiled
• n/a Bumped version number in setup.py and package.json

FYI: @edx/masters-devs-gta