Move file type policy README to be with the config

The config file was moved in http://crrev.com/c/2076416, but the readme
did not go along with it. The README is now the only file in
chrome/browser/resources/safe_browsing, and should be with the config it
describes.

Change-Id: I5503f3c2c788d6e42405be0e53d3db45895dbe8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4754019


Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Commit-Queue: thefrog <thefrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180465}