docs: add readme with maintenance instructions

README.md

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+# nimentas-sasup
+
+## Maintenance
+
+This repository has 2 primary branches:
+
+- `main` - the branch with the latest, stable version of the code. This is the branch that is deployed to niployments.
+- `upstream` - the branch that is used to merge in changes from the upstream repository. The `upstream` branch should be merged into the `main` branch whenever the code is considered stable.
+
+To update the `upstream` branch, you can [click here](https://github.com/NIAEFEUP/nimentas-sasup/compare/upstream...ttoino:ementas:main).