Add a README file in each arch-bom owned edx-platform app

[robrap]

Best practice is to have a README for each app. Since we maintain edx-platform, we should role model.

Acceptance Criteria:

• [ ] Create README for /common/djangoapps/third_party_auth
• [ ] Review apps in the ownership spreadsheet and verify they're done.

Additional Notes:

• ./openedx/core/djangoapps/auth_exchange (DONE)
• /openedx/core/djangoapps/oauth_dispatch (DONE)
• Are there others we own?