Update README.md

Clearing up the configuration steps.
Adding a sample .env file.
Merge branch 'master' of github.com:mozilla/login.webmaker.org
Update README.md

Describing how to use the .env.sample file.
Fixed hardcoded AUDIENCE path in sso.js
Foo-fixes
Fixed hardcoded AUDIENCE path in sso.js
Restricted cross-origin requests to the contents of process.env.ALLOWED_DOMAINS
Restricted postmessage to ALLOWED_DOMAINS config value
Merge branch '_dev' of https://github.com/ksedge/login.webmaker.org into _dev
Merge pull request #4 from ksedge/_dev

Fixes #1, #2, #3 - Fixing security bugs
Quick refactor of the routes
Merge pull request #5 from rossbruniges/refactor

Quick refactor of the routes