Add a skeletal PR template (#2226)

e.g. https://github.com/flutter/flutter/blob/master/.github/PULL_REQUEST_TEMPLATE.md

But I feel weird adding a checklist

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+_Replace this paragraph with a description of what this PR is changing or
+adding, and why._
+
+Closes #ISSUE