//ios/chrome and //ios/web_view were missing README files which are
important to differentiate the products in each of those directories.

Additionally, update README files within //ios missing the appropriate
`.md` extension. This is important so that the contents of the file are
rendered in code search.

Bug: None
Change-Id: I1529602a5854b0c89e1fe808313ff36652367bab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3915109


Reviewed-by: Éric Noyau <noyau@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1051255}
NOKEYCHECK=True
GitOrigin-RevId: c9cf79801f8700ebf2748ad7f1f514bff2cb3347