Update test section in README.md about pytest

[gschwind]

Overview

fix documentation to update instruction for testing

Related Issue / Discussion

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage remained the same at 81.222% when pulling 645294c on gschwind:fix-readme-001 into c8d1fe1 on geopython:main.