Add Fig as autocomplete suggestion (#2574)

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

README.md

@@ -357,6 +357,11 @@ See [Advanced configuration](#advanced-configuration) for details and more confi
 
    **Proxy note**: If you use a proxy, export `http_proxy` and `https_proxy`, too.
 
+   In MacOS, you might also want to install [Fig](https://fig.io/) which
+provides alternative shell completions for many command line tools with an
+IDE-like popup interface in the terminal window.
+(Note that their completions are independent from Pyenv's codebase
+so they might be slightly out of sync for bleeding-edge interface changes.)
 
 ### Restart your shell