Add a note for fish shell users (close #1203)

README.md

@@ -205,8 +205,9 @@ easy to fork and contribute any changes back upstream.
     ```sh
     $ echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n  eval "$(pyenv init -)"\nfi' >> ~/.bash_profile
     ```
-    **Zsh note**: Modify your `~/.zshenv` file instead of `~/.bash_profile`.
-    **Ubuntu and Fedora note**: Modify your `~/.bashrc` file instead of `~/.bash_profile`.
+    - **Zsh note**: Modify your `~/.zshenv` file instead of `~/.bash_profile`.
+    - **fish note**: Use `pyenv init - | source` instead of `eval (pyenv init -)`.
+    - **Ubuntu and Fedora note**: Modify your `~/.bashrc` file instead of `~/.bash_profile`.
 
     **General warning**: There are some systems where the `BASH_ENV` variable is configured
     to point to `.bashrc`. On such systems you should almost certainly put the abovementioned line