Update packages/desktop-cli/README.md with instruction to init and de…

…velop the desktop app project

packages/desktop-cli/README.md

@@ -1 +1,50 @@
 # `@stlite/desktop-cli`
+
+Convert your [Streamlit](https://streamlit.io/) application into a desktop app with [stlite](https://github.com/whitphx/stlite) runtime, a [Pyodide](https://pyodide.org/)-based Wasm-port of Streamlit.
+
+## How to create a Streamlit desktop app
+
+1. Create a new NPM project with the following `package.json`. Edit the `name` field.
+   ```json
+   {
+     "name": "xxx",
+     "version": "0.1.0",
+     "main": "./build/electron/main.js",
+     "scripts": {
+       "dump": "dump-stlite-desktop-artifacts",
+       "serve": "NODE_ENV=\"production\" electron .",
+       "pack": "electron-builder --dir",
+       "dist": "electron-builder",
+       "postinstall": "electron-builder install-app-deps"
+     },
+     "build": {
+       "files": ["build/**/*"],
+       "directories": {
+         "buildResources": "assets"
+       }
+     },
+     "devDependencies": {
+       "@stlite/desktop-cli": "^0.12.0",
+       "electron": "20.2.0",
+       "electron-builder": "^23.3.3"
+     }
+   }
+   ```
+2. Run `npm install` or `yarn install`.
+3. Create `streamlit_app` directory.
+   - Any directory name can be used here.
+4. Create `streamlit_app/streamlit_app.py`.
+   - Change the directory name if you used a different name in the previous step.
+5. Write your Streamlit app code in `streamlit_app/streamlit_app.py`.
+   - The file name `streamlit_app.py` is not configurable now.
+6. Optionally, you can add more contents in the directory, including `pages/*.py` for multi-page apps, any data files, and so on.
+7. Run `npm run dump streamlit_app` or `yarn dump streamlit_app`. The command argument `streamlit_app` is the directory name of the Streamlit app you have created in the previous steps. Change it if you used a different name.
+   - If installing some packages is needed, use `-r` option like `npm run dump streamlit_app -r <PackageName1> ... <PackageNameN>`.
+   - This command creates `./build` directory. It includes
+     - The stlite bare app files.
+     - `streamlit_app` directory copied from the one you created in the previous steps.
+     - `site-packages-snapshot.tar.gz` that includes the installed package files.
+8. Run `npm run serve` or `yarn serve` for preview.
+   - This command is just a wrapper of `electron` command as you can see at the `"scripts"` field in the `package.json`. It launches Electron and starts the app with `./build/electron/main.js`, which is specified at the `"main"` field in the `package.json`.
+9. Run `npm run dist` or `yarn dist` for packaging.
+   - This command just calls [`electron-builder`](https://www.electron.build/) (see the `"scripts"` field in the `package.json`) and it simply bundles the `./build` directory created in the step above. If you want to customize the built app, e.g., setting the icon, follow its instruction.