meta: add pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,17 @@
+Thanks in advance for your contribution to Discord.Net!
+
+Before opening a pull request, please consider the following:
+
+Does your changeset adhere to the Contributing Guidelines?
+
+Does your changeset address a specific issue or idea? If not, please
+break your changes up into multiple requests.
+
+Have your changes been previously discussed with other members
+of the community? We prefer new features to be vetted through
+an issue or a discussion in our Discord channel first; bug-fixes
+and other small changes are generally fine without prior vetting.
+
+Please remove this section, and include a summary of your changes
+below.
+---