Update README zsh completion for bundled installer

The README's zsh section points to bin/aws_zsh_completer.sh, which
is shipped via Package Manager installs (e.g. pip) but not by the
bundled installer (zip/pkg). Bundled-installer users following the
README hit "No such file or directory" when sourcing it.

Keep the existing pip instructions and add a parallel block for
bundled installs using the bashcompinit + complete -C approach
documented in the AWS CLI User Guide. Both methods now coexist
without removing the legacy path that pip users still rely on.

Fixes aws#8957

Author: adev-code

README.rst

@@ -87,14 +87,27 @@ For tcsh::
 
 You should add this to your startup scripts to enable it for future sessions.
 
-For zsh please refer to ``bin/aws_zsh_completer.sh``.  Source that file, e.g.
-from your ``~/.zshrc``, and make sure you run ``compinit`` before::
+For zsh (Package Manager, e.g. pip installs) please refer to 
+``bin/aws_zsh_completer.sh``.  Source that file, e.g. from your 
+``~/.zshrc``, and make sure you run ``compinit`` before::
 
     $ source bin/aws_zsh_completer.sh
 
 For now the bash compatibility auto completion (``bashcompinit``) is used.
 For further details please refer to the top of ``bin/aws_zsh_completer.sh``.
 
+For zsh (bundled installer, e.g. zip/pkg), use ``aws_completer``
+directly with ``bashcompinit``::
+
+    $ autoload bashcompinit && bashcompinit
+    $ autoload -Uz compinit && compinit
+    $ complete -C '/usr/local/bin/aws_completer' aws
+
+Replace ``/usr/local/bin/aws_completer`` with the path returned by
+``which aws_completer`` if different. Add these commands to your
+``~/.zshrc`` to enable completion in future sessions.
+
+
 ---------------
 Getting Started
 ---------------