add CONTRIBUTING

CONTRIBUTING

+# Contributing
+
+How to set up your dev environment to work on the grib processor.
+
+First, create a virtual environment like in the [readme](/README.md#setup).
+
+Then, install ruff and mypy.
+
+Before pushing commits run:
+
+`ruff format grib_processor.py`
+
+`ruff check grib_processor.py`
+
+`mypy grib_processor.py`
+
+>Note: mypy will give an error about untyped imports, this can be ignored.