Add FAQ about config file errors

[pmaxted]

In [1]: from pycheops.core import setup_config
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
File /opt/anaconda3/envs/cheops/lib/python3.9/site-packages/pycheops/__init__.py:55
     54 try:
---> 55     psf_path = path.join(data_path, config['psf_file']['psf_file'])
     56 except KeyError:

File /opt/anaconda3/envs/cheops/lib/python3.9/configparser.py:963, in RawConfigParser.__getitem__(self, key)
    962 if key != self.default_section and not self.has_section(key):
--> 963     raise KeyError(key)
    964 return self._proxies[key]

KeyError: 'psf_file'

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
Cell In[1], line 1
----> 1 from pycheops.core import setup_config

File /opt/anaconda3/envs/cheops/lib/python3.9/site-packages/pycheops/__init__.py:57
     55     psf_path = path.join(data_path, config['psf_file']['psf_file'])
     56 except KeyError:
---> 57     raise KeyError("Run pycheops.core.setup_config(overwrite=True) to"
     58                    " update your config file.")
     59 if not path.isfile(pfile) or (path.getmtime(pfile) < path.getmtime(psf_path)):
     60     radius = 33  # Aperture radius in pixels

KeyError: 'Run pycheops.core.setup_config(overwrite=True) to update your config file.'