better doc of process

Author: drammock

doc/sphinxext/related_software.py

@@ -5,6 +5,10 @@
 1. Add it to the MNE-installers if possible, and it will automatically appear.
 2. If it's on PyPI and not in the MNE-installers, add it to the PYPI_PACKAGES set.
 3. If it's not on PyPI, add it to the MANUAL_PACKAGES dictionary.
+
+If PyPI or manual, also add package name to `related_software.txt` or
+`related_software_nodeps.txt` so that it's installed at doc-build time (for package
+metadata querying).
 """
 
 # Authors: The MNE-Python contributors.