hterm embed.md: Document installKeyboard
hterm embed.md: Document installKeyboard

Change-Id: Ic440b2532c7c7417d9c376161653ca41f24566f7
Reviewed-on: https://chromium-review.googlesource.com/415026
Reviewed-by: Rob Ginda <4419ac4e35d191308e944202d542b192188656d1@chromium.org>
Tested-by: Rob Ginda <4419ac4e35d191308e944202d542b192188656d1@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm