Create PULL_REQUEST_TEMPLATE

PULL_REQUEST_TEMPLATE

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+### Description
+
+add a description of your changes here...
+
+### Rationale
+
+tell us why we need these changes...
+
+### Example
+
+add an example CLI or API response...
+
+### Changes
+
+Notable changes: 
+* add each change in a bullet point here
+* ...
+
+### Preflight checks
+
+- [ ] build passed (`make build`)
+- [ ] tests passed (`make test`)
+- [ ] manual transaction test passed
+
+### Already reviewed by
+
+...
+
+### Related issues
+
+... reference related issue #'s here ...