Add tip to set SRC_ENDPOINT environment variable after login

[dadlerj]

Feel free to close/replace this if another approach is preferred

Test plan

Tested locally

✔︎ Authenticated as dadlerj on https://sourcegraph.com


✔︎ Authenticated with OAuth credentials

💡 Tip: To use this endpoint in your shell, run:

   export SRC_ENDPOINT=https://sourcegraph.com