Docs: link to zsh completing for emulator

This links to zsh-users/zsh-completions, which includes a completion
script for the Android emulator command line tool.

Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I527c5da96af560ad9853c00050fc5e7dcc36705d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128786Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754779}

docs/android_emulator.md

@@ -62,6 +62,11 @@ Refer to: https://developer.android.com/studio/run/emulator-commandline.html.
 Ctrl-C will gracefully close an emulator.
 ***
 
+*** promo
+**Tip:** zsh users can add https://github.com/zsh-users/zsh-completions to
+provide tab completion for the `emulator` command line tool.
+***
+
 ### Basic Command Line Use
 ```shell
 $ # List virtual devices that you've created: