Update README.md for electron apps (polkadot-js#3362)

* Update README.md

* Add info on Electron app to the top-level README.

Co-authored-by: Krzysztof Jelski <krzysztof@ethworks.io>

Author: vanruch

README.md

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 ```
+
+## Desktop App
+
+The main advantage of using Desktop App is that it by default stores encrypted accounts on the filesystem instead of browser's local storage.
+Local storage is susceptible to attacks using XSS (Cross-Site Scripting). There's no such risk when with files stored on disk.
+
+The desktop app uses the [Electron](https://www.electronjs.org/) framework. It provides the same features as web app, the only difference
+being different account storage.
+
+The accounts are stored in the following directories:
+* Mac: `~/Library/Application Support/polkadot-apps/polkadot-accounts`
+* Linux: `~/.config/polkadot-apps/polkadot-accounts` (or `$XDG_CONFIG_HOME/polkadot-apps/polkadot-accounts` if `$XDG_CONFIG_HOME` is defined)
+* Windows: `%APPDATA%\polkadot-apps\polkadot-accounts`
+
+For more details on the desktop app, head over to [Electron package README](https://github.com/polkadot-js/apps/blob/master/packages/apps-electron/README.md).

packages/apps-electron/README.md

@@ -1,3 +1,17 @@
 # @polkadot/apps-electron
 
-WARNING: This is not deemed stable yet for external use.
+Desktop Polkadot apps client for Windows, Mac and Linux. 
+
+## Installation
+
+[Download here](https://github.com/polkadot-js/apps/releases/latest) latest versions for Windows, Mac and Linux.
+
+## Development and testing
+
+Contributions are welcome!
+
+Follow steps described [here](https://github.com/polkadot-js/apps#development) to setup the project.
+
+* Run `yarn start:electron` to start the app in development mode. You will possibly see the `Not Found / 404` message. It's ok, just wait for the build to finish and refresh pressing `Ctrl+R`.
+* Run `yarn test` to run tests
+* Run `yarn packElectron:(mac|linux|windows)` with the OS you want to build for to create the app executable. Find the packages in `packages/apps-electron/release`.