Update install instructions for using GMT master (#279)
Update install instructions for using GMT master (#279)

Point Windows users to use the GMT6 binaries provided by Joaquim.
Instructions to create conda environment and install dependencies.
Fix the description of GMT_LIBRARY_PATH, since GMT is no longer installed via conda.

Fixes #213.
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python