Add pull request template to sub packages (#876)

* Add pull request template to sub packages

* Fix doc generator

* Fix copyright year

.github/pull_request_template.md

@@ -0,0 +1,24 @@
+**PLEASE READ THIS ENTIRE MESSAGE**
+
+Hello, and thank you for your contribution! Please note that this repository is
+a read-only split of `GoogleCloudPlatform/google-cloud-php`. As such, we are
+unable to accept pull requests to this repository.
+
+We welcome your pull request and would be happy to consider it for inclusion in
+our library if you follow these steps:
+
+* Clone the parent client library repository:
+
+```sh
+$ git clone [email protected]:GoogleCloudPlatform/google-cloud-php.git
+```
+
+* Move your changes into the correct location in that library. Library code
+belongs in `src/Bigtable`, and tests in `tests/*/Bigtable`.
+
+* Push the changes in a new branch to a fork, and open a new pull request
+[here](https://github.com/GoogleCloudPlatform/google-cloud-php).
+
+Thanks again, and we look forward to seeing your proposed change!
+
+The Google Cloud PHP team