GDAL required for pip install to succeed

[cjtu]

On Ubuntu 22.04 without a system-wide gdal installation, pip install planetarypy fails at gdal:

...
      gdal_config_error: [Errno 2] No such file or directory: 'gdal-config'
      
      Could not find gdal-config. Make sure you have installed the GDAL native library and development headers.
      [end of output]

Maybe add a link to how to get gdal in the installation instructions?

Alternatively, pip installing planetarypy into a conda env after conda install -c conda-forge gdal also works.