Create PR template

.github/pull_request_template.md

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+# Before Making this Pull Request
+
+- Please review the [Style Guide](https://github.com/electro-smith/DaisySP/blob/master/doc/style_guide.pdf), and do your best to make sure your code fits the design ethos of the library.
+    - Please be sure to run `util/fix_style.sh` and commit any relevant changes.
+    - As appropriate, please be sure to document your code in line.
+- Please add any references to issues this PR fixes using the # tag.
+- Please add a relevant label (for example bug, documentation, etc.)
+- Fill out the sections below as relevant.
+- Thank you for your contribution!
+
+# Description
+
+(Please include a summary of the changes and the related issue. Please also include relevant motivation and context.)
+
+# Testing
+
+(How was this code tested to confirm it works properly? Please do not make a PR for untested code.)
+
+# Demos / Example Code
+
+(If relevant, please add any video or audio demos, or example code for your PR.)