Added a README.md for third_party/blink/renderer/modules/manifest.

Links to it from new README files in other related directories.

Change-Id: I9dcca96de0e8e0b4fc885727efac800bdcc2abd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275976Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784334}

content/browser/manifest/README.md

+Please see blink/renderer/modules/manifest's [README
+file](../../../third_party/blink/renderer/modules/manifest/README.md).

third_party/blink/common/manifest/README.md

+Please see blink/renderer/modules/manifest's [README
+file](../../renderer/modules/manifest/README.md).

third_party/blink/public/mojom/manifest/README.md

+Please see blink/renderer/modules/manifest's [README
+file](../../../renderer/modules/manifest/README.md).

third_party/blink/renderer/modules/manifest/README.md

+This directory contains (most of) the implementation of the [Web App Manifest
+specification](https://www.w3.org/TR/appmanifest/).
+
+The internal representation of the manifest, as returned by the manifest parser,
+is defined as a Mojo struct in
+[blink/public/mojom/manifest/manifest.mojom](third_party/blink/public/mojom/manifest/manifest.mojom).
+The [manifest parser](manifest_parser.cc) is responsible for converting the
+manifest from its JSON representation to that Mojo struct.
+
+The interpretation of those manifest members is typically done elsewhere,
+usually in the browser layer (not in Blink), since most of the Manifest features
+describe how the browser interacts with the operating system.