docs: add link to security.txt release tarball verification.

Author: rouilj

doc/installation.txt

@@ -74,6 +74,10 @@ program you can run ``python demo.py`` instead.)
 .. _install the source:
 
 1. ``python3 -m pip download roundup``
+
+   * (If you are really impatient, you probably don't want to
+     `verify the downloaded file <security.html#verify-source-tarball>`_.)
+
 2. ``tar -xzvf roundup-2.2.0.tar.gz``
 
    * if you don't have a tar command, ``python3 -c "import tarfile, sys; tarfile.open(sys.argv[1]).extractall();" roundup-2.2.0.tar.gz`` can be used.