Add instruction how to enable bash/zsh completion to website

bash and zsh require different syntax for enabling shell completion than
fish. That difference was accounted for in README, but not in docs (and,
consequently, on website).

docs/advanced.rst

@@ -394,6 +394,10 @@ To enable completion in fish, add this to your config::
 
     eval (pipenv --completion)
 
+Alternatively, with bash or zsh, add this to your config::
+
+    eval "$(pipenv --completion)"
+
 Magic shell completions are now enabled!
 
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