Create README and document monorepo structure

Author: anttimaki

README.md

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+# Thunderstore UI
+
+Monorepo containing Next.js frontend for [thunderstore.io](https://thunderstore.io)
+and reusable UI components.
+
+## Monorepo Setup
+
+- [`yarn` workspaces](https://classic.yarnpkg.com/en/docs/workspaces/) manages
+  the packages in the monorepo (see `packages` key in base `package.json` file)
+  and handles dependency installation/deduplication.
+- [`preconstruct`](https://preconstruct.tools/) automates building and linking
+  the packages within the monorepo. Instead of using relative paths, local
+  packages can be imported as if they were installed via a package manager.
+  - Packages can be linked locally by running `yarn preconstruct dev`, but this
+    is handled automatically by `postinstall` hook, so developers don't need to
+    worry about it.
+
+```
+// first time setup
+git clone git@github.com:thunderstore-io/thunderstore-ui.git thunderstore-ui
+cd thunderstore-ui
+yarn install
+
+// start Next.js dev server
+yarn workspace @thunderstore/nextjs dev
+```
+
+That's it. Changes done to `apps/nextjs` and `packages/components` should both
+be automatically visible at [http://localhost:3000/].
+
+### Adding dependencies
+
+To add new dependencies to existing packages, simply run something like:
+
+```
+yarn workspace @thunderstore/components add react-table @types/react-table
+```
+
+### Adding a new package
+
+To add a completely new package, start by creating the following file structure:
+
+```
+// packages/greeter/package.json:
+{
+  "name": "@thunderstore/greeter",
+  "version": "1.0.0",
+  "description": "Example package"
+}
+
+// packages/greeter/src/index.tsx:
+import React from "react";
+
+export const Greeter: React.FC = () => <p>Hello, world!</p>;
+```
+
+To add some required fields to the new package's `package.json`, run
+`yarn preconstruct init` and allow modifying `package.json` when asked.
+
+To install dependencies, if any, run e.g.
+`yarn workspace @thunderstore/greeter add react react-dom @types/react`.
+
+To "install" the new package to Next.js app, update the `dependencies` section
+in `apps/nextjs/package.json` with `"@thunderstore/greeter": "^1.0.0",`.
+
+Then run `yarn` one more time to let Preconstruct work its magic. After that the
+new package should be usable in the Next.js app by simply importing it:
+
+```
+import { Greeter } from "@thunderstore/greeter";
+```
+
+### About VS Code...
+
+VS Code may have problem detecting installed packages in this monorepo/workspace
+setup. Installing
+[Monorepo Workspace extension](https://marketplace.visualstudio.com/items?itemName=folke.vscode-monorepo-workspace)
+may solve them.