Add documentation URL and trove classifiers in setup.cfg

setup.cfg

@@ -7,6 +7,25 @@ description = the MESSAGEix integrated assessment model
 long_description_content_type = text/markdown
 long_description = file:README.md
 url = http://github.com/iiasa/message_ix
+project_urls =
+    Documentation = https://docs.messageix.org/
+classifiers =
+    Development Status :: 5 - Production/Stable
+    Intended Audience :: Developers
+    Intended Audience :: Science/Research
+    License :: OSI Approved :: Apache Software License
+    Natural Language :: English
+    Operating System :: OS Independent
+    Programming Language :: Python
+    Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Programming Language :: R
+    Topic :: Scientific/Engineering
+    Topic :: Scientific/Engineering :: Information Analysis
+
 
 [options]
 packages = message_ix