docs(PR): Add PR template

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+*(Describe the issue you are solving and some info about how you accomplished it)*
+
+## Test Plan
+##### How can reviewers test your change manually?
+(Please describe)
+##### What automated tests did you add? If none, please state why.
+(Please describe)
+
+## Security Impact
+Please answer the questions below about your PR. 
+If yes, please explain and add `cedar-team/security` as reviewers.
+
+##### Add/modify authentication, authorization, encryption, or HTTP headers?
+Yes/No
+##### Add/modify any PII or PHI data fields?
+Yes/No
+##### Add new third parties the app interacts with (e.g., Nordis/Stripe)?
+Yes/No
+##### Ask patients to enter a new kind of data, (e.g. insurance capture)?
+Yes/No
+##### Impact any of the other [Security Engineering Practices](https://careportal.atlassian.net/wiki/spaces/SEC/pages/1192690295/Security+Engineering+Practices)?
+_Protect sensitive data, Validate your inputs, Know your user, Establish trust boundaries, Keep good records, Expect vulnerabilities, Follow least privilege_
+
+Yes/No