message: Update DEVELOPERS.md with PR instructions for 3rd parties
diff: ```bash
  $ yarn install
  -$ yarn lerna bootstrap
  ```
  # Documentation
  API and SDK documentation can be found at: https://platform.bitgo.com/.
  +# Pull Requests
  +
  +Pull requests from third parties are more than welcome! When creating a pull
  +request, a BitGo engineer will be automatically added as a reviewer.
  +
  +If you notice that the integration test job never runs, don't worry - that's
  +expected. The integration tests use a secret testing password, and so we can't
  +run these tests on untrusted code (PRs from third parties) without reviewing
  +first. Once your code is passing unit tests and has an initial review, a BitGo
  +engineer will re-create your PR and sign your commits, and link back to the
  +original PR.
  +
  # Things to Avoid
  You should not be using the `tsc` command in terminal. Doing so WITHOUT a tsconfig file
  (which informs tsc where to install files) would lead to translation files
  