Add fish install oneliner (#322)

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

README.md

@@ -34,6 +34,12 @@ From inside that directory you can:
     $ git clone https://github.com/pyenv/pyenv-virtualenv.git $(pyenv root)/plugins/pyenv-virtualenv
     ```
 
+    For the Fish shell:
+
+    ```sh
+    $ git clone https://github.com/pyenv/pyenv-virtualenv.git (pyenv root)/plugins/pyenv-virtualenv
+    ```
+
 2. (OPTIONAL) **Add `pyenv virtualenv-init` to your shell** to enable auto-activation of virtualenvs. This is entirely optional but pretty useful. See "Activate virtualenv" below.
 
     ```sh