Create pull_request_template.md

Author: HeshamMegid

.github/pull_request_template.md

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+## Summary of Changes
+
+
+<!-- For pull requests that add new APIs, please make sure to go through this checklist. 
+For other types of pull requests, please remove it.-->
+## Checklist
+- [ ] In addition to adding the new API to Dart, I have also added its implementation to both iOS and Android.
+- [ ] I have added at least 1 unit test in Dart for the new API and made sure the tests pass locally.
+- [ ] I have updated [README.md](https://github.com/Instabug/Instabug-Flutter/blob/master/README.md) to add the new API.