Installation instructions - pygdal version

[edoddridge]

Submitted as part of a review for JOSS.

The instructions include reference to a specific version of pygdal

Install using pip, the correct version. Note you may need to increase the minor version number, e.g. from 2.1.3 to 2.1.3.3

pip install pygdal==2.1.3

I needed to modify this to pip install pygdal==2.2.3.3. It might be better to state that the version number needs to match the output of gdal-config --version (with the possible addition of a minor version number), rather than pinning it to a specific version that will go stale quickly.