Doco: add a Shell Completion doc

This describes how to configure your shell to use Homebrew-managed completion
definitions. Needed because this is a manual step the user has to do.

docs/README.md

@@ -6,6 +6,7 @@
 - [Installation](Installation.md)
 - [Frequently Asked Questions](FAQ.md)
 - [Common Issues](Common-Issues.md)
+- [`brew` Shell Completion](Shell-Completion.md)
 
 - [Tips and Tricks](Tips-N'-Tricks.md)
 - [Bottles (binary packages)](Bottles.md)

docs/Shell-Completion.md

@@ -0,0 +1,44 @@
+# Homebrew Shell Completion
+
+Homebrew comes with completion definitions for the `brew` command. Some packages also provide completion definitions for their own programs.
+
+`zsh`, `bash` and `fish` are currently supported. (Homebrew provides `brew` completions for `zsh` and `bash`; `fish` provides its own `brew` completions.)
+
+You must configure your shell to enable the completion support. This is because the Homebrew-managed completions are stored under `HOMEBREW_PREFIX`, which your system shell may not be aware of, and because it is difficult to automatically configure `bash` and `zsh` completions in a robust manner, so the Homebrew installer cannot do it for you.
+
+## Configuring Completions in `bash`
+To make Homebrew's completions available in `bash`, you must source the definitions as part of your shell startup. Add the following to your `~/.bashrc` file:
+
+```sh
+if type brew 2&>/dev/null; then
+  for completion_file in $(brew --prefix)/etc/bash_completion.d/*; do
+    source "$completion_file"
+  done
+fi
+```
+
+## Configuring Completions in `zsh`
+To make Homebrew's completions available in `zsh`, you must get the Homebrew-managed zsh site-functions on your `$FPATH` before initializing `zsh`'s completion facility. Add the following to your `~/.zshrc` file:
+
+```sh
+if type brew &>/dev/null; then
+  FPATH=$(brew --prefix)/share/zsh/site-functions:$FPATH
+fi
+```
+
+This must be done before `compinit` is called. (Note: if you are using Oh My Zsh, it will call `compinit` for you, so this must be done before you call `oh-my-zsh.sh`.)
+
+You may also need to forcibly rebuild `zcompdump`:
+
+```sh
+  rm -f ~/.zcompdump; compinit
+```
+
+Additionally, if you receive "zsh compinit: insecure directories" warnings when attempting to load these completions, you may need to run this:
+
+```sh
+  chmod go-w "$(brew --prefix)/share"
+```
+
+## Configuring Completions in `fish`
+No configuration is needed in `fish`. Friendly!