rbfu: add README

plugins/rbfu/README.md

+# rbfu plugin
+
+This plugin starts [rbfu](https://github.com/hmans/rbfu), a minimal Ruby version
+manager, and adds some useful functions.
+
+To use it, add `rbfu` to the plugins array in your zshrc file:
+```zsh
+plugins=(... rbfu)
+```
+
+**Note: `rbfu` is deprecated and should no longer be used.**
+
+## Functions
+
+- `rbfu-rubies`: lists all installed rubies available to rbfu.
+
+- `rvm_prompt_info`: shows the Ruby version being used with rbfu.