Add pull request template

.github/pull_request_template.md

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+# Pull request guidelines
+
+Before working on a pull request, create an issue explaining what you want to contribute. This ensures that your pull request won't go unnoticed, and that you are not contributing something that is not suitable for the project. Once a core developer has set the `help wanted` label on the issue, you can submit a pull request. The pull request description should reference the original issue.
+
+Your pull request must:
+
+ * Address a single issue or add a single item of functionality.
+
+ * Contain a clean history of small, incremental, logically separate commits,
+   with no merge commits.
+
+ * Use clear commit messages.
+
+ * Be possible to merge automatically.
+
+When you rea making PR that adds new spatial features to OpenMapTiles schema, please make also PR for at least one of our GL styles to show it on the map. Visual check is crucial.