Add zsh completion script

Fixes #163

--
MOS_MIGRATED_REVID=95542929

Author: schroederc

site/docs/install.md

@@ -134,7 +134,7 @@ $ export PATH="$PATH:$HOME/bazel/output"
 
 You can also add this command to your `~/.bashrc` file.
 
-## Getting bash completion
+### Getting bash completion
 
 Bazel comes with a bash completion script. To install it:
 
@@ -145,4 +145,25 @@ Bazel comes with a bash completion script. To install it:
    you and simply insert `source /path/to/bazel-complete.bash` in your
    `~/.bashrc` file (under OS X, put it in your `~/.bash_profile` file).
 
+### Getting zsh completion
+
+Bazel also comes with a zsh completion script. To install it:
+
+1. Add this script to a directory on your $fpath:
+
+    ```
+    fpath[1,0]=~/.zsh/completion/
+    mkdir -p ~/.zsh/completion/
+    cp scripts/zsh_completion/_bazel ~/.zsh/completion
+    ```
+
+2. Optionally, add the following to your .zshrc.
+
+    ```
+    # This way the completion script does not have to parse Bazel's options
+    # repeatedly.  The directory in cache-path must be created manually.
+    zstyle ':completion:*' use-cache on
+    zstyle ':completion:*' cache-path ~/.zsh/cache
+    ```
+
 For more information, see [Getting started](getting-started.html).