Make GitHub Issue/PR templates #5
Description
We already have a very simple .github/PULL_REQUEST_TEMPLATE.md file for SwiftGen.
Let's generalize that for all repos to have markdown templates both for issues and PR.
They should help contributors:
- For issues: ensure that they chose the right repo for submitting issue
- For Mrs: not forget to credit themselves in the CHANGELOG and format the CHANGELOG properly
- Ensure that they ran the appropriate tests (including the ones run by the CI but not by Xcode, like linting and test-compiling the template outputs)
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