Clarify README for fzf environment variables.

[cohml]

fzf is extremely customizable using environment variables. Because your project simply calls fzf in the background, your package can also benefit from this customizability.

Unfortunately, this fact about pyfzf must be discovered via trial and error or else by just reading the source code. Much better for the user would be to simply make it explicit in the README.

Hence this PR simply updates the README to clarify this fact about fzf environment variables, including a simple example.