message: Update yarn unit test instruction in DEVELOPER.md
diff: # Installing
  +In the BitGoJS/ directory:
  ```bash
  $ yarn install
  ```
  @@ -48,12 +49,18 @@ Or just the modules which have changed since master:
  $ yarn run unit-test-changed
  ```
  -Or the file that contains a specific string:
  +Or the file that contains a specific string
  + - if you are a specific module:
  +
  ```
  $ yarn run unit-test --grep"keyword"
  ```
  +- if you are in the BitGoJs/ top-level directory
  +```
  +$ yarn run unit-test -- -- --grep "keyword"
  +```
  **Note:** Each module's output will be prefixed with it's name, but it may be difficult to unmangle the test output.
  To aid with evaluating test results, and especially test failures, a fancy test report is generated by the CI system and uploaded to S3.
  