Include details about how to use TrichromeWebView prerelease channels in
the documentation.

Bug: 1016502
Change-Id: Ifb7b8878f6a85d57e01a637922036f8870923d48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929519
Commit-Queue: Richard Coles <torne@chromium.org>
Auto-Submit: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718322}