Add pull-request note for the first-time contributors (#8016)

Here is a text for the initial response to first-time contributors
asking them to perform local testing of their PR.  It is hoped that
this process would encourage a due level of attention to the
correctness and quality of proposed changes.

---------

Co-authored-by: Michael Hucka <mhucka@google.com>

Author: pavoljuhas

.github/PR_TEMPLATE/first_time_contributor.md

@@ -0,0 +1,19 @@
+Thank you for your interest in contributing to Cirq.
+
+In order to discourage unchecked automatically-generated content,
+we ask our first time contributors to follow the guidelines at
+https://quantumai.google/cirq/dev/development and perform
+local testing of their changes to ensure the PR is ready for review.
+
+Please confirm with a short comment on this PR when this is done.
+The comment should state the Python version and the operating system
+used for testing, for example,
+
+> Running check/all passes all tests locally using Python 3.13
+> and a Debian Linux OS.  Ready for review.
+
+If some of the tests present too much difficulty to run locally or
+to resolve, please note them in your comment.  That said, we
+expect that code formatting, lint checks and Python unit tests
+for the changed files would be all done and addressed before
+it is our turn to review the contribution.