docs: note in Changelog that v3→v4 requires adding eslintConfig

[mimecuvalo]

I came across this when trying to upgrade my company's create-react-app from v3→v4. The migration path doesn't note that you will lose ESLint capabilities unless you specify eslintConfig explicitly in your package.json going forward.

Historical progression of ESLint changes in Webpack, to clarify how we got here:

• removal of original extend and addition to template.json: https://github.com/facebook/create-react-app/pull/9587/files
• referencing of /base instead of /index: https://github.com/facebook/create-react-app/pull/9640/files#diff-8e25c4f6f592c1fcfc38f0d43d62cbd68399f44f494c1b60f0cf9ccd7344d697R366

This was also noticed by other developers in these bugs:
#9791
#10212

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