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This adds a developer guide for the WebView shell. This covers building
and installing, but also shows how to interact with the shell via the
commandline.

This cross-links to the shell's folder (adding a README.md), but keeps
primary documentation under android_webview/docs/ to improve
discoverability.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I6216bb70a42989910044cfc56e56314a4e2bc781
Reviewed-on: https://chromium-review.googlesource.com/c/1490909
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636952}