message: Add some information on testing to the contribution guidelines.
  Issue
diff: @@ -72,6 +72,8 @@ We strongly recommend that contributors:
  * We will be adding internal checks that automate this requirement by
  matching the PR description to the regexp: `(Fixes|Issue) #`
  +1. Unit tests are critical to a healthy codebase. PRs without tests should be the exception rather than the norm. And contributions to improve, simplify, or make the unit tests more exhaustive are welcome! Please refer to [this guideline](https://google.github.io/eng-practices/review/developer/small-cls.html#test_code) on how test code and writing small PRs should be reconciled.
  +
  ## Guidlines for Specific Contribution Categories
  We provide some additional guidelines for different categories of contributions.
  @@ -86,6 +88,7 @@ fixing a bug needs a bigger architectural change.
  [TF Lite Micro Github issue](https://github.com/tensorflow/tensorflow/issues/new?labels=comp%3Amicro&template=70-tflite-micro-issue.md)
  to determine the scope of the bug fix.
  1. Send a PR (if that is determined to be the best path forward).
  +1. Bugfix PRs should be accompanied by a test case that fails prior to the fix and passes with the fix to both validate that the fix works as expected and prevent future regressions.
  ### Reference Kernel Implementations
  @@ -156,3 +159,4 @@ and more scalable.
  Having said that, we still invite feature requests via
  [TF Lite Micro Github issues](https://github.com/tensorflow/tensorflow/issues/new?labels=comp%3Amicro&template=70-tflite-micro-issue.md)
  to determine if the requested feature aligns with the TFLM roadmap.
  +
  