Added RTD to project_urls

Added a link to RTD to project_urls so it's easier to find the documentation from the pypi site.

Author: lennartk

setup.cfg

@@ -1,6 +1,8 @@
 [metadata]
 name = jsonschema
 url = https://github.com/Julian/jsonschema
+project_urls =
+    Docs = https://python-jsonschema.readthedocs.io/en/stable/
 description = An implementation of JSON Schema validation for Python
 long_description = file: README.rst
 author = Julian Berman