Add pull request template (#1588)

Author: dakinggg

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,29 @@
+# What does this PR do?
+
+<!--
+Please briefly describe your change, including what problem the change fixes, and any context
+necessary for understanding the change
+-->
+
+# What issue(s) does this change relate to?
+
+<!--
+Please include any issues related to this pull request, including 'Fixes' if the issue is resolved
+by this pull request.
+Example:
+- Fixes #42
+- Related to #1234
+-->
+
+# Before submitting
+- [ ] Have you read the [contributor guidelines](https://github.com/mosaicml/composer/blob/dev/CONTRIBUTING.md)?
+- [ ] Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
+- [ ] Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
+- [ ] Did you update any related docs and document your change?
+- [ ] Did you update any related tests and add any new tests related to your change? (see [testing](https://github.com/mosaicml/composer/blob/dev/CONTRIBUTING.md#running-tests))
+- [ ] Did you run the tests locally to make sure they pass?
+- [ ] Did you run `pre-commit` on your change? (see the `pre-commit` section of [prerequisites](https://github.com/mosaicml/composer/blob/dev/CONTRIBUTING.md#prerequisites))
+
+<!--
+Thanks so much for contributing to composer! We really appreciate it :)
+-->