[react] Add comment about removed index signature

[frenic]

The index signature for React.CSSProperties was removed to enable closed typing. In this PR I just added a comment about that and how to use module augmentation to add CSS custom properties or a index signature of their own.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).
• Provide a URL to documentation or source code which provides context for the suggested changes: <>
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

[frenic]

cc @pelotom

[typescript-bot]

@frenic Thank you for submitting this PR!

🔔 @johnnyreilly @bbenezech @pzavolinsky @digiguru @ericanderson @morcerf @tkrotoff @DovydasNavickas @onigoetz @theruther4d @guilhermehubner @JoshuaKGoldberg @jrakotoharisoa - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[pelotom]

“...or add an* index signature...”

[frenic]

Thanks 👍

[typescript-bot]

@frenic One or more reviewers has requested changes. Please address their comments. I'll be back once they sign off or you've pushed new commits. Thank you!

[pelotom]

Maybe also good to mention that a cheap alternative, esp. for missing properties where you don't need to reference it in multiple places, is casting:

<div style={{ ['overflowAnchor' as any]: 'auto' }}>
  ...
</div>

[frenic]

I added that to the README at csstype. Thanks 👍

[pelotom]

How about adding a comment to the effect of "If you find that these typings are missing a valid CSS property or value, please check whether an issue has already been filed for it at https://github.com/frenic/csstype/issues and if not, create one."

[frenic]

@pelotom I changed the comment a bit and added a link to the README of csstype where they can find exampes and more information. It's easier to maintain that README rather than this comment.

[johnnyreilly]

Thanks!

[AdamTReineke]

This helped me debug a compile error today after updating React, thank you!