Update install instructions

[seisman]

Description of proposed changes

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

Fixes #213.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[leouieda]

Thanks @seisman 👍 Just a few minor changes.

[seisman]

@leouieda I've accepted your changes and merged master branch.

[leouieda]

Thanks @seisman! We should actually try setting up AppVeyor with those binaries. It will probably fail and we might have problems testing different GMT versions on Windows and Linux.