Add a pull request template. (#370)

* Add a pull request template.

.github/pull_request_template.md

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+## Changes in this PR
+*Clearly and concisely summarize the changes you are making. Bullet points are completely okay. Please be specific, saying "improves X" is not enough!*
+
+### Why are you making these changes?
+*Explain why these changes are necessary. Link to GitHub issues here with the format `fixes: #XXX` to indicate this PR resolves the issue.*
+
+### Are any changes breaking? (IMPORTANT)
+*Will merging this PR change `poly`'s API in a non-backwards-compatible manner?*
+
+*Examples of breaking changes:*
+* *Removing a method from a struct.*
+* *Deleting/moving a package.*
+* *Adding a method to an interface (client may have made their own     implementation of this interface, and adding a method to the     interface could cause client's implementation to no longer satisfy     the interface).*
+
+*Examples of non-breaking changes:*
+* *Adding a method to a struct.*
+* *Adding a function.*
+* *Fixing a bug in a function/method.*
+* *Creating a new package.*
+
+## Pre-merge checklist
+*All of these must be satisfied before this PR is considered
+ready for merging. Mergeable PRs will be prioritized for review.*
+
+* [ ] New packages/exported functions have docstrings.
+* [ ] New/changed functionality is thoroughly tested.
+* [ ] New/changed functionality has a function giving an example of its usage in the associated test file. See `primers/primers_test.go` for what this might look like.
+* [ ] Changes are documented in `CHANGELOG.md` in the `[Unreleased]` section.
+* [ ] All code is properly formatted and linted.
+* [ ] The PR template is filled out.