Bring back the mention of Debian packages

Signed-off-by: Richard Laager <rlaager@wiktel.com>

README.rst

@@ -12,7 +12,7 @@ Building Locally
 Install Prerequisites
 ~~~~~~~~~~~~~~~~~~~~~
 
-::
+The dependencies are available via pip::
 
    # For Debian based distros
    sudo apt install pip3
@@ -23,6 +23,11 @@ Install Prerequisites
    # Add ~/.local/bin to your $PATH, e.g. by adding this to ~/.bashrc:
    PATH=$HOME/.local/bin:$PATH
 
+Debian 11 (“testing”) / Ubuntu 20.04 or later have the dependencies packaged::
+
+   sudo apt install python3-sphinx python3-sphinx-issues \
+       python3-sphinx-rtd-theme
+
 Build
 ~~~~~