docs(typescript-estree): add the link for AST Alignment Tests

[sosukesuzuki]

I was curious about the differences between Babel and typescript-estree ASTs, so I tried to look into it, but I was confused because the link to the code does not exist.

This PR adds link for AST Alignment Tests to README.

[typescript-eslint]

Thanks for the PR, @sosukesuzuki!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[bradzacher]

thanks!