[infra] add a PR template for the sdk repo

Change-Id: I875b5cb72b42a115be694a303bb080f0545e48bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/302120
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+*Thanks for your contribution! Please replace this paragraph with a description
+of what this PR is changing or adding and why. List any relevant issues, and
+review the contribution guidelines below.*
+
+---
+
+### Contribution guidelines:
+
+- See our
+  [contributor guide](https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md)
+  for general expectations for PRs.
+- Larger or significant changes should be discussed in an issue before creating
+  a PR.
+- Contributions to our repos should follow the
+  [Dart style guide](https://dart.dev/guides/language/effective-dart) and use
+  `dart format`.
+
+- [ ] I’ve reviewed the contributor guide and applied the relevant portions to
+      this PR.
+
+Note that this repository uses Gerrit for code reviews. Your pull request will
+be automatically converted into a Gerrit CL and a link to the CL written into
+this PR. The review will happen on Gerrit but you can also push additional
+commits to this PR to update the code review.
+
+---