Update patts docs to describe editing ebuild too

docs/accessibility/patts.md

 # The Chrome OS PATTS speech synthesis engine
 
 Chrome OS comes with a speech synthesis engine developed internally at Google
 called PATTS. It's based on the same engine that ships with all Android devices.
 
 ## Building from source
 
 This is for Googlers only.
 
 Visit [http://go/chrome-tts-blaze](http://go/chrome-tts-blaze)
@@ skipping 63 matching lines @@
 After that, reboot your Chromebook and verify that speech works.
 
 To upload the change, use repo upload, something like this:
 
 ```
 git commit -a
   BUG=chromium:12345
   TEST=Write what you tested here
 repo upload .
 ```
+
+## Ebuild
+
+Note that sometimes you'll have to update the ebuild file that
+takes the patts data files and installs them, unzipping the .nexe
+files in the process.
+
+For example, you'll need to edit the ebuild if you add or remove
+a language code, or if you add or remove a file that needs to be
+installed as part of the extension.
+
+To update the ebuild, edit this file:
+
+```
+/third_party/chromiumos-overlay/chromeos-base/common-assets/common-assets-9999.ebuild
+```
+
+If you need to land changes to both common-assets and chromiumos-assets,
+upload the changes separately and then make them depend on one another
+using this syntax in the changelog:
+
+```
+CQ-DEPEND=CL:12345
+```
+
+Note that you can (and often should) have two changes depend on one another
+so they'll land atomically.
+
+