Add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+
+<!--
+Thanks for contributing to Altair!
+
+Read the guidelines below and then delete this comment before submitting your PR.
+
+- Describe the purpose of the changes in PR, so that it is easy to understand the implication of the suggested changes.
+    - Here is a helpful article about writing effective PR descriptions https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3.
+- Include unit tests and documentation for new feautres
+- Ensure that the title is a concise [semantic commit message](https://www.conventionalcommits.org/) (e.g. "feat: Add embed_options to charts").
+    - Append `!` if the change is breaking (e.g. "fix!: Error when embed_options are None")
+-->