Add some other links which will show up on PyPI.

Author: Julian

setup.cfg

@@ -2,7 +2,9 @@
 name = jsonschema
 url = https://github.com/Julian/jsonschema
 project_urls =
-    Docs = https://python-jsonschema.readthedocs.io/en/latest/
+    Documentation = https://python-jsonschema.readthedocs.io/en/latest/
+    Source = https://github.com/Julian/jsonschema
+    Issues = https://github.com/Julian/jsonschema/issues/
 description = An implementation of JSON Schema validation for Python
 long_description = file: README.rst
 long_description_content_type = text/x-rst