[GPT-1] README.md: Document Tailwind CSS usage

There is no mention of [Tailwind CSS](https://tailwindcss.com/) in `README.md`, which should be rectified with some links and basic explanations to guide new developers.

<sub>[GPT-1](https://linear.app/advisorysg/issue/GPT-1/readmemd-document-tailwind-css-usage)</sub>