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Update the documentation to run brltty on linux as well. This is sometimes hard to [re]discover.

Change-Id: Ia6cfefa2af9465762fc31c8077fec7310edca083
Reviewed-on: https://chromium-review.googlesource.com/c/1294512Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601811}