Add READMEs to deep scanning main directories

Bug: 1109240
Change-Id: I70af6c0371edffbaf4bf108cb332683de4c77bdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498880
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821332}

chrome/browser/enterprise/connectors/README.md

+This directory contains code used by Chrome Enterprise Connectors. This
+includes:
+  * Code to handle Connector policies.
+  * Code specific to uploaded content analysis.
+  * Common utilities used by Connector code.
+
+Code shared with user downloads cloud scanning should be added to
+`//chrome/browser/safe_browsing/cloud_content_scanning/` instead.
+
+Code specific to user downloads cloud scanning should be added to
+`//chrome/browser/safe_browsing/download_protection/` instead.

chrome/browser/safe_browsing/cloud_content_scanning/README.md

+This directory includes cloud scanning code that is shared across user uploads
+and user downloads scanning features. This includes:
+  * Chrome Enterprise Connectors that scan content.
+  * Chrome Advanced Protection Program file download scans.
+
+Code specific to user downloads cloud scanning should be added to
+`//chrome/browser/safe_browsing/download_protection/` instead.
+
+Code specific to user uploads cloud scanning or Chrome Enterprise Connectors
+should be added to `//chrome/browser/enterprise/connectors/` instead.

chrome/browser/safe_browsing/download_protection/README.md

+This directory includes code that scans downloaded content for security reasons.
+This includes:
+  * Safe Browsing metadata checks.
+  * Chrome Advanced Protection Program file download scans.
+  * Chrome Enterprise Connectors file download scans.
+
+Code shared with user uploads cloud scanning should be added to
+`//chrome/browser/safe_browsing/cloud_content_scanning/` instead.
+
+Code specific to user uploads cloud scanning or Chrome Enterprise Connectors
+should be added to `//chrome/browser/enterprise/connectors/` instead.