feat: added PR template

Updated docs to reflect it.

.github/PULL_REQUEST_TEMPLATE/PR_TEMPLATE.md

@@ -0,0 +1,25 @@
+Description of PR...
+
+## Changes
+
+- Item 1
+- Item 2
+
+## Checklist
+
+- [ ] Unit tests
+- [ ] Documentation
+
+## References
+
+(optional)
+
+Include **important** links regarding the implementation of this PR.
+This usually includes and RFC or an aggregation of issues and/or individual conversations
+that helped put this solution together. This helps ensure there is a good aggregation
+of resources regarding the implementation.
+
+```text
+Fixes #85, Fixes #22, Fixes username/repo#123
+Connects #123
+```

README.md

@@ -13,6 +13,7 @@ This repo holds Speckle's:
 - Default [Contribution Guidelines](.github/CONTRIBUTING.md),
 - README template (you're reading it now),
 - Default [Issue Template](.github/ISSUE_TEMPLATE/ISSUE_TEMPLATE.md),
+- Default [Pull Request Template](.github/PULL_REQUEST_TEMPLATE/PR_TEMPLATE.md),
 - OSS License (Apache 2.0)
 
 Either copy paste the parts that are useful in existing repos, or use this as a base when creating a new repository.