Add PR template

Issue #40

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+# Hello and thank you for contributing to slinky
+
+Before opening your PR please make sure to do the following before:
+
+- Write a message explaining your changes.
+- Add an entry to [`CHANGELOG.md`](../CHANGELOG.md) about your changes (under
+  the `Unreleased` category).
+- Make sure `cargo test` passes locally.
+- Run both `cargo clippy --fix` and `cargo fmt`.
+- If this PR fixes an issue then add `fixes #issue_number` is the PR description.
+  - This will close the issue when the PR is merged.
+  - If it doesn't fix an issue but it is somewhat related to an issue/PR then
+    mention the `#issue_number` on the PR description (without the `fixes`).
+- Delete this line and everything above it.