Add a pull request template

This is to highlight contributing guidelines and in particular call
out that Googlers must send us Piper CLs.

PiperOrigin-RevId: 523127713
Change-Id: I38e219d811adc5f14c7aeaa952d5f25de75198f5

.github/PULL_REQUEST_TEMPLATE.md

+Thank you for your contribution to Abseil!
+
+Before submitting this PR, please be sure to read our [contributing
+guidelines](https://github.com/abseil/abseil-cpp/blob/master/CONTRIBUTING.md).
+
+If you are a Googler, please also note that it is required that you send us a
+Piper CL instead of using the GitHub pull-request process. The code propagation
+process will deliver the change to GitHub.

CONTRIBUTING.md

@@ -75,9 +75,9 @@ will be expected to conform to the style outlined
 
 ## Guidelines for Pull Requests
 
-*   If you are a Googler, it is preferable to first create an internal CL and
-    have it reviewed and submitted. The code propagation process will deliver
-    the change to GitHub.
+*   If you are a Googler, it is required that you send us a Piper CL instead of
+    using the GitHub pull-request process. The code propagation process will
+    deliver the change to GitHub.
 
 *   Create **small PRs** that are narrowly focused on **addressing a single
     concern**. We often receive PRs that are trying to fix several things at a