First time setup docs touchups

@DSASanFrancisco/portal-members

(apologies, re-opening !204 (closed) as this merge request after finding out that slashes in branch names are a no-no)

While I was setting up this repo for the first time, I ran into a few small confusing things, and edited the README with the answers I got from #tech channel on slack:

1. Make it clear exactly which variables should be edited in your .env for first-time setup
2. Small clarifying comment about the behavior of make
3. Fix the tips for how to connect to the MySQL database (it needs a password!)