Add ios/web/public/find_in_page/README.md

README says: "This directory contains API to search for text on a web
page. This directory does not support any UI API except content
highlighting."

Bug: 942244
Change-Id: I8e683ac1a99783becf39fbc7f8bd61017c017641
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619209
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661343}

ios/web/public/find_in_page/README.md

+This directory contains API to search for text on a web page. This directory does not support any UI API except content highlighting.