Update accessibility documentation.

docs/accessibility/brltty.md

+# BRLTTY in Chrome OS
+
+Chrome OS uses the open-source [BRLTTY](http://mielke.cc/brltty/)
+library to provide support for refreshable braille displays.
+
+We typically ship with a stable release build of BRLTTY plus some
+cherry-picked patches.
+
+## Updating BRLTTY or adding a patch
+
+First, follow the public
+[Chromium OS Developer Guide](http://www.chromium.org/chromium-os/developer-guide) to check out the source.
+At a minimum you'll need to create a chroot.
+You do not need to build everything from source.
+You do need to start the devserver.
+
+Next, flash your device to a very recent test build. Internally at Google
+you can do this with the following command when the dev server is running,
+where CHROMEBOOK_IP_ADDRESS is the IP address of your Chromebook already
+in developer mode, and $BOARD is your Chromebook's board name.
+
+```cros flash ssh://CHROMEBOOK_IP_ADDRESS xbuddy://remote/$BOARD/latest-dev/test```
+
+The BRLTTY files can be found in this directory:
+
+```third_party/chromiumos-overlay/app-accessibility/brltty```
+
+The first thing you'll need to do is edit the ebuild symlink to change the
+revision number. The real file is something like brltty-5.4.ebuild,
+but the revision will be something like brltty-5.4-r5.ebuild. You'll need
+to increment it.
+
+To increment it from r5 to r6, you'd do something like this:
+
+```
+rm brltty-5.4-r5.ebuild
+ln -s brltty-5.4.ebuild brltty-5.4-r6.ebuild
+git add brltty-5.4-r6.ebuild
+```
+
+The changes we make are all patches against a stable release of brltty.
+To add a new patch, put it in the files/ directory and reference it in
+brltty.bashrc
+
+Once you're done adding patches or making other changes, flash it to your
+device like this:
+
+```
+emerge-$BOARD brltty
+cros deploy CHROMEBOOK_IP_ADDRESS brltty
+```
+
+After that, reboot your Chromebook and verify that brltty works.
+
+To upload a change, use repo, something like this:
+
+```
+repo start <branch_name> .
+git commit -a
+  BUG=chromium:12345
+  TEST=Write what you tested here
+repo upload .
+```
+
+Note that you shouldn't need to run cros_workon.