Pull Request template (#1485)

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for contributing to Lychee! We only accept PR to the master branch.
+
+In addition, please describe the benefit to end users; the reasons it does not break any existing features, etc.
+
+If you're pushing a Feature:
+- Title it: "This new feature"
+- Describe what the new feature enables
+- Add tests to test the new feature.
+- Ensure it doesn't break any existing features.
+
+If you're pushing a Fix:
+- Title it: "Fixes the bug name"
+- If it is a fix an an existing issue start the description with `fixes #xxxx` where xxxx is the issue number.
+- Describe how it fixes in a few words.
+- Add a test that triggered the bug before fixing it.
+- Ensure it doesn't break any feature.
+
+All Pull Requests run with extensive tests for stable and latest versions of PHP. 
+Ensure your tests pass or your PR may be taken down.
+
+Additionally you can run `make phpstan` and `make formatting` on your own machine as they will be required to pass for your PR to be accepted.
+
+Don't worry if your code styling isn't perfect! php-cs-fixer will automatically create a pull request with any style fixes. This allows us to focus on the content of the contribution and not the code style.
+-->