Add a pull request template

This pull request template was mainly added so that the second bullet point in [this PR comment](python-discord/api#17 (comment)) can be addressed.

Author: doublevcodes

.github/PULL_REQUEST_TEMPLATE/pull_request.md

@@ -0,0 +1,19 @@
+## Summary
+This PR introduces [ brief summary ] in accordance with site#[ issue number ]
+
+## Description of changes
+- `foo` was refactored to `bar`
+- The baz feature was implemented
+- ...
+
+### I confirm I have:
+- [ ] Joined the [Python Discord community](discord.gg/python)
+- [ ] Read the [Code of Conduct](https://www.pydis.com/pages/code-of-conduct) and agree to it
+- [ ] I have discussed implementing this feature on the relevant service (Discord, GitHub, etc.)
+
+### If I have changed any models (edited files under the `site/apps/api/models` directory), I ensure I have:
+- [ ] Opened a PR updating the model on the [API GitHub Repository](https://github.com/python-discord/api)
+
+**OR**
+
+- [ ] Opened an issue on the [API GitHub Repository](https://github.com/python-discord/api) explaining what changes need to be made