Update readme with missing feature flag.

[nicolas-sabbatini]

The readme file is missing the codegen feature flag.

Description

I just added the missing feature flag.

Motivation and Context

If you read the readme and tray to use the lib in a wasm project, the build is going to fail, and you are not going to know why.

How Has This Been Tested?

It displays correctly in the GitHub README.

Checklist:

• Acknowledged that by making this pull request I release this code under an MIT/Apache 2.0 dual licensing scheme.
• My code follows the code style of this project.
• If my change required a change to the documentation I have updated the documentation accordingly.
• I have updated the content of the book if this PR would make the book outdated.
• I have added tests to cover my changes.
• My code is used in an example.

[nicolas-sabbatini]

I believe that the last 4 point of the checklist don't apply.