clarify --mpl-generate-hash-library location in README.rst

[bjlittle]

The current README.rst doesn't explicitly mention where the hash library will be created i.e.,

At present the provided hash library file will be created at the root of the package.

I'm happy to add this clarification, if you agree this is useful?

[Cadair]

Does it always generate in the root of the repo or can you specify a path in that cli flag?

[ConorMacBride]

There is documentation in plugin.py but this should be added to the readme.

pytest-mpl/pytest_mpl/plugin.py

Lines 100 to 102 in e387618

	group.addoption('--mpl-generate-hash-library',
	help="filepath to save a generated hash library, relative "
	"to location where py.test is run", action='store')