Add pr template

(cherry picked from commit 8aa037edca4ecf3a9a5a779ccae0412c1ff8d55a)

.github/pull_request_template.md

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+# Contribute to the phpDocumentor Guides
+
+## Go to the mono-repository
+
+This project is developed in the mono-repository `phpDocumentor Guides <https://github.com/phpDocumentor/guides>`__.
+The repository you are currently in gets auto-created by splitting the mono-repository. You **must not** contribute
+to this repository directly but always to the mono-repository linked above.
+
+## Create Issues
+
+*   If you find something missing or something is wrong in this library, you are welcome to write an issue
+    describing the problem: `Issues on GitHub <https://github.com/phpDocumentor/guides/issues>`__.
+*   If you can, please try to fix the problem yourself.
+
+# Make changes (create pull requests)
+
+See the `Contribution chapter <https://docs.phpdoc.org/components/guides/guides/contributions/index.html>`__ in the
+`Documentation` <https://docs.phpdoc.org/components/guides/guides/index.html>`__.