chore: add PR template (#46)

Add documentation update checklist item to PR template,
to ensure that changes/updates to the SDK are properly documented

.github/pull_request_template.md

@@ -0,0 +1,12 @@
+# Summary
+<!--- A short summary about what this PR is doing. -->
+# Why the changes
+<!--- State the reason/context for the change. -->
+
+# Things worth calling out
+<!--- Give useful tips/gotchas/trade-offs made to the reviewers. -->
+
+# Before merging
+- [ ] ***For Immutable developers:*** Do the changes in this PR require documentation updates? Please refer to this [SDK documentation guide](https://immutable.atlassian.net/wiki/spaces/PPS/pages/1916994017/SDK+documentation+guide) and list links to documentation update PRs (they don't have to be merged) below (or write `N/A`):
+    - [ ] *Add documentation update 1*
+    - [ ] *Add documentation update 2*