Ability to customize zsh in ~/.yadr/custom/zsh folder

* Split zsh config into snippets in zsh/ folder, similar to vim
 * Feature implementation by @kylewest

* kylewest/zsh:
  Documentation for customizing ZSH.
  Only init fasd if it is installed.
  Removed brew and github plugins.
  updated .gitignore for new zsh paths.
  simplifying paths for zsh customization.
  added zsh aliases. Moved other files but left symlinks for backwards compatibility.
  automatically load RVM or rbenv.
  updated .gitignore
  adding sample zsh configs.
  added directories and code for custom zsh configuration.

README.md

@@ -172,6 +172,21 @@ mnemonic aliases. Please feel free to edit them:
  * Bash style ctrl-R for reverse history finder
  * Fuzzy matching - if you mistype a directory name, tab completion will fix it
 
+### How To Customize ZSH
+
+YADR allows you to completely customize your ZSH without having to fork and maintain the project. Here's how it works: YADR will
+source (include) any files matching the pattern `*.before.zsh` or `*.after.zsh` in the `custom/zsh` directory. `before` files are
+useful for setting the theme and plugins. `after` files allow you to override options set by YADR, define your own aliases, etc.
+
+To make your life easier, create a `zsh` folder in your Dropbox (or as a git repo) and symlink it into `~/.yadr/custom`. Do it like this:
+
+```bash
+ln -s ~/Dropbox/path/to/zsh ~/.yadr/custom/zsh
+```
+
+Create as many `whatever.before.zsh` or `whatever.after.zsh` files as you need within the `zsh` directory. Please see `custom/zsh.sample` for
+an example.
+
 
 ## Pry