Create template for PRs

Just adding a little structure to ensure PRs can get processed effectively. 

See https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

Author: tig

pull_request_template.md

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+## Description
+
+Please include a terse summary of the change or which issue is fixed.
+  
+If an Issue was fixed, write "Fixes #issuenumber" as this will help Github automatically close that issue once the PR is merged.
+
+## Temrminal.Gui Pull Request checklist:
+
+- [ ] My code follows the [style guidelines of Terminal.Gui](https://github.com/gui-cs/Terminal.Gui/blob/develop/.editorconfig) - if you use Visual Studio, hit `CTRL-K-D` to automatically reformat your files before committing.
+- [ ] Mhy code follows the [Terminal.Gui library design guidelines](https://github.com/gui-cs/Terminal.Gui/blob/develop/CONTRIBUTING.md)
+- [ ] I ran `dotnet test` before commit
+- [ ] I have performed a self-review of my own code
+- [ ] I have commented my code, particularly in hard-to-understand areas
+- [ ] I have made corresponding changes to the API documentation (using `///` style comments)
+- [ ] My changes generate no new warnings
+- [ ] I have checked my code and corrected any poor grammar or misspellings
+- [ ] My pull request generate no conflicts with `develop` branch
+- [ ] I tested my code on at least Windows and Linux 
+- [ ] I conducted basic QA to assure all features are working