Added a HOWTO page release based source edits.

conf.py
-------
   * Exclude all variants of python virtual env from traversal.
   * Exclude __pycache__ from document traversal.

index.rst
---------
   * Create a top level index link/page for release howto(s).

overview/contributing.rst
-------------------------
   * Document version based script edits required at time of release.
   * This URL will be used by repo:ci-management/vars/installVoltctl
     to display self-documenting help.

*/urls
------
   * Added placeholders and resources for documenting future topics.

Chaser edits:
-------------
   o Bump VERSION string.
   o 'stern' URL in unrelated troublteshooting.rst doc failed link check:
      + github repo no longer exists.
      + add replacement github/stern/stern

Change-Id: I2f0f15eb5ba7bd6817806296928964df5259b21a
diff --git a/howto/test/by/commit b/howto/test/by/commit
new file mode 100644
index 0000000..6e0176b
--- /dev/null
+++ b/howto/test/by/commit
@@ -0,0 +1,6 @@
+# Placeholder for future docs.
+# HOWTO: Trigger testing using patchset keywords.
+
+https://docs.voltha.org/master/testing/voltha_test_automation.html#per-patchset-verification-jobs
+
+# [EOF]