Update README about the docs

This directory contains the source files of the Fedify docs.  The docs are

written in Markdown format and are built with [VitePress].

In order to build the docs locally, you need to install [Bun] first.  Then you

can run the following commands (assuming you are in the *docs/* directory):

In order to build the docs locally, you need to install [Node.js] and [pnpm]

first.  Then you can run the following commands (assuming you are in

the *docs/* directory):

~~~~ bash

bun install

bun dev

pnpm install

pnpm run dev

~~~~

Once the development server is running, you can open your browser and navigate

to *http://localhost:5173/* to view the docs.

[VitePress]: https://vitepress.dev/

[Bun]: https://bun.sh/

[Node.js]: https://nodejs.org/

[pnpm]: https://pnpm.io/