more info (at the least) needed for new contributors

[itcarroll]

I ran into a few issues setting up for development and testing. I think the solutions will mostly be documentation, but some config too.

1. The contributing documentation does not describe the environment variables you need to set up for pytest to discover tests. It seems like at least EARTHDATA_USERNAME and EARTHDATA_PASSWORD must be set.
2. Although I created the earthaccess-dev environment, I had to mamba install libmagic for pytest to discover tests.
3. I cannot a number of integration tests to pass on the main branch. Some documentation might be all that's needed? I seemed to also need environment variables EARTHDATA_TEST_USERNAME and EARTHDATA_TEST_PASSWORD, but that's not all. Here is one failure instance below that I haven't debugged.

   tests/integration/test_auth.py::test_auth_can_read_earthdata_env_variables failed: def t 
   test_auth_can_read_earthdata_env_variables():
           activate_environment()
           auth = earthaccess.login(strategy="environment")
   >       logger.info(f"Current username: {auth.username}")
   E       AttributeError: 'Auth' object has no attribute 'username'
   
   tests/integration/test_auth.py:44: AttributeError
   

4. The pre-commit CI test isn't documented, and seems to do more than is covered by the make format lint noted in the contributing documentation.
5. Guidance on when an enhancement might be more suitable for python-cmr than earthaccess.