add more notes to rosdoc2

pages/rosdoc2.md

@@ -2,3 +2,27 @@
 
 Documentation for ROS 2 packages can be built with
 [rosdoc2](https://github.com/ros-infrastructure/rosdoc2).
+
+When a ``doc`` job is configured in the ``rosdistro``, the resulting
+documentation is uploaded to ``docs.ros.org``.
+The design document for
+[per package documentation](http://design.ros2.org/articles/per_package_documentation.html)
+lays out the directory structure.
+
+## Linking to Other Packages
+
+
+This is a dirty hack, but appears to be the only way to have the table of contents
+link to another package's documentation without hard-coding the distro. The trailing
+``http://`` changes how Sphinx processes the link:
+
+```rst
+.. toctree::
+   :maxdepth: 2
+
+   other_package <../other_package/index.html#http://>
+```
+
+An example of this can be seen in the documentation for
+[image_pipeline](http://docs.ros.org/en/rolling/p/image_pipeline/), where we want
+to link to the documentation for each of the packages within the metapackage.