message: Add "contribution welcome" in Bug template
  This will add a candidate contribution claim in the Bug Template (we could add this also in other template if you like)
  /cc
diff: @@ -28,11 +28,14 @@ You can also obtain the TensorFlow version with:
  1. TF 1.0: `python -c "import tensorflow as tf; print(tf.GIT_VERSION, tf.VERSION)"`
  2. TF 2.0: `python -c "import tensorflow as tf; print(tf.version.GIT_VERSION, tf.version.VERSION)"`
  -
  **Describe the current behavior**
  **Describe the expected behavior**
  +**[Contributing](https://www.tensorflow.org/community/contribute)**
  +- Do you want to contribute a PR? (yes/no):
  +- Briefly describe your candidate solution (if contributing):
  +
  **Standalone code to reproduce the issue**
  Provide a reproducible test case that is the bare minimum necessary to generate
  the problem. If possible, please share a link to Colab/Jupyter/any notebook.
  