[Deprecations] (4) Update README to reflect devtools codegen

Now that devtools is using codegen many steps in adding new deprecations
can be skipped. Let's reflect this in the instructions.

This CL is part of a series:
(1) Add deprecation.json5 file
(2) Add existing deprecations to deprecation.json5
(3) Copy deprecation.json5 into devtools_frontend via update flow
(4) Update README to reflect devtools codegen
(5) Replace browser_protocol.pdl enum
(6) Codegen c++ code

Bug: 1411444
Change-Id: I543c589cf734f6da2d27154e6b7a1cf677540367
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4226916
Reviewed-by: Mike Taylor <miketaylr@chromium.org>
Commit-Queue: Ari Chivukula <arichiv@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1102299}