Update README with component mapping setting

[khiga8]

Context

In #283 we introduced a config that allows custom components to be mapped to an HTML element for our JSX rules.

Changes

This adds a README that explains how the component mapping works. I marked this feature as experimental because it is new and subject to change.

This PR also updates the parserOptions for the react config. Let me know if this looks wrong though.

Note

I plan to cut a new release following this PR.

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[khiga8]

I believe this is the configuration we want for React linting based on language options guide and the config in primer-react/recommended.

[owenniblock]

Great docs (especially the note on is the as isn't defined. I can't comment on the configuration change unfortunately, is there an easy way to test it before we merge?

[lindseywild]

Thanks for adding these docs! ✨

[khiga8]

@owenniblock I recently just learned that you can install npm packages from a github branch! I ran the following on dotcom:

npm i git://github.com/github/eslint-plugin-github.git#kh-add-docs-for-component-config  --save

The lint ran fine and caught some issues. But also I didn't run into issues with the previous config so I'm not really sure what I'm supposed to expect. This looks more right though...

[khiga8]

I guess I'll test this in memex too! If it looks good I shall merge this.