update package metadata (colcon#11)

README.rst

@@ -0,0 +1,4 @@
+colcon-cmake
+============
+
+An extension for `colcon-core <https://github.com/colcon/colcon-core>`_ to support `CMake <https://cmake.org>`_ projects.

setup.cfg

@@ -1,8 +1,10 @@
 [metadata]
 name = colcon-cmake
 version = attr: colcon_cmake.__version__
-url = https://colcon.github.io
-download_url = https://github.com/colcon/colcon-cmake/releases
+url = https://colcon.readthedocs.io
+project_urls =
+    Changelog = https://github.com/colcon/colcon-cmake/milestones?state=closed
+    GitHub = https://github.com/colcon/colcon-cmake/
 author = Dirk Thomas
 author_email = [email protected]
 maintainer = Dirk Thomas
@@ -19,6 +21,7 @@ classifiers =
     Topic :: Software Development :: Build Tools
 license = Apache License, Version 2.0
 description = Extension for colcon to support CMake packages.
+long_description = file: README.rst
 keywords = colcon
 
 [options]