docs: mention prettier in eslint-config README

[exKAZUu]

Summary

The combination of eslint (v7.0.0), @react-native-community/eslint-config (v1.1.0), and flow-typed (v3.1.0) causes the following error (ESLint couldn't find the plugin "eslint-plugin-prettier") because flow-typed (v3.1.0) depends on prettier: ^1.19.1 (c.f. https://github.com/flow-typed/flow-typed/blob/master/cli/package.json#L38).
To deal with the error, developers should install prettier (v2.x) directly in devDependencies.

Oops! Something went wrong! :(

ESLint: 7.0.0

ESLint couldn't find the plugin "eslint-plugin-prettier".

(The package "eslint-plugin-prettier" was not found when loaded as a Node module from the directory "/Users/exkazuu/Projects/test".)

It's likely that the plugin isn't installed correctly. Try reinstalling by running the following:

    npm install eslint-plugin-prettier@latest --save-dev

The plugin "eslint-plugin-prettier" was referenced from the config file in ".eslintrc.js » @react-native-community/eslint-config".

If you still can't figure out the problem, please stop by https://gitter.im/eslint/eslint to chat with the team.

Changelog

[Internal] [Changed] - @react-native-community/eslint-config README recommends developers to install prettier directly

Test Plan

This PR changes only README, so tests are not required.

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[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	6,781,984	0
android	hermes	armeabi-v7a	6,446,172	0
android	hermes	x86	7,169,068	0
android	hermes	x86_64	7,060,399	0
android	jsc	arm64-v8a	8,949,832	0
android	jsc	armeabi-v7a	8,606,409	0
android	jsc	x86	8,780,029	0
android	jsc	x86_64	9,356,975	0

Base commit: c8d678a

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	809,568	0

Base commit: c8d678a

[matt-oakes]

Thanks for this. I think this makes sense. People should already be getting a warning about a missing peer dependency as the ‘eslint-plugin-prettier’ package has that defined.

@cpojer Can you merge this in?

[cpojer]

Thank you!

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @exKAZUu in 0a2bb71.

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