Add preload readme

plugins/README.md

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+## What's it?
+
+This folder is the source code of the preload scripts and built-in plugins.
+
+### Tech stacks
+
+![](https://skillicons.dev/icons?i=vite,ts,solidjs,tailwind)
+
+- **Vite** - dev server, build tool and bundler.
+- **TypeScript** - typesafe!
+- **SolidJS** - high performance JSX embedded views, no vDOM!
+- **Tailwind** - no more CSS!
+
+## Developing
+
+We have two folders in `src`
+
+- preload: pofyfill, loader and hook scripts
+- views: SolidJS app - a successor of the previous @default plugin
+
+First, you need to build the core module in `Debug` mode.
+
+Second, install dependencies:
+
+```
+pnpm install
+```
+
+Then run the dev server:
+
+```
+pnpm dev
+```
+
+Now start your **League Client** to get the result. Additionally, you can open
+https://localhost:3001 in a web browser to test the views.
+
+Any changes of views will perform hot-replacement in DOM. With preload script
+changes, it will reload entire client.
+
+If you need to modify the `preload` only, run dev build and reload the client
+manually.
+
+```
+pnpm build-dev
+```
+
+> Don't touch vite.config.ts without knowing it.
+
+## Building production
+
+Run build
+
+```
+pnpm build
+```
+
+To embed the scripts into the core module, you should build it in `Release`
+mode.