Update README.md

[yenchenLiu]

Currently, when follow the doc to build will fail because the relative path is wrong.

$ python rdpy/setup.py install
running install
running bdist_egg
running egg_info
creating rdpy.egg-info
writing rdpy.egg-info/PKG-INFO
writing dependency_links to rdpy.egg-info/dependency_links.txt
writing requirements to rdpy.egg-info/requires.txt
writing top-level names to rdpy.egg-info/top_level.txt
writing manifest file 'rdpy.egg-info/SOURCES.txt'
package init file 'rdpy/__init__.py' not found (or not a regular file)
error: package directory 'rdpy/core' does not exist