ci(coverage): updating CONTRIBUTING to reflect CI constraints (#239)

Author: guzmud

CONTRIBUTING.md

@@ -19,6 +19,15 @@ developing features or fixing bugs yourself.
 * If you are interested in contributing code, fork the repository, create a
   branch, and open a pull request.
 
+## Code quality and testing
+
+In order to maintain a certain standard regarding code quality, this repository CI will run various tools against expectations:
+- `isort` will be used to check for proper imports sorting, and the CI will fail if the checks fail
+- `black` will be used to check for proper code formatting, and the CI will fail if the checks fail
+- `flake8` will be used to check for proper code styling, and the CI will fail if the checks fail
+- `pytest` will be used to run various levels of testing, and the CI will fail if the tests fail
+- `coverage` with `codecov` will be used to check for proper code coverage, and the CI will fail if the level is below 80% for the current diff (a warning will be emitted if the level falls below 80% for the overall project)
+
 <!-- filigran-conventions:start -->
 ## Commit, pull request & issue conventions