Make PR template a bit more welcoming

Author: killercup

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,9 @@
-- [ ] Tests created for any new feature or regression tests for bugfixes.
+Thank you for taking the time to contribute to this project!
+
+To ease reviewing your changes and making sure we don't forget anything,
+please take a minute to check the following (remove lines that don't apply):
+
+- [ ] I have created tests for any new feature, or added regression tests for bugfixes.
 - [ ] `cargo test` succeeds
-- [ ] `cargo +nightly clippy` succeeds
-- [ ] `cargo +nightly fmt -- --write-mode=diff` succeeds
+- [ ] Clippy is happy: `cargo +nightly clippy` succeeds
+- [ ] Rustfmt is happy: `cargo +nightly fmt` succeeds