docs: update README.md

added shields, introduction, development

README.md

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+# Secure Spreadsheets
+
+<div align="center">
+
+[![GitHub issues and feature requests](https://img.shields.io/github/issues/standardnotes/secure-spreadsheets.svg)](https://github.com/standardnotes/secure-spreadsheets/issues/)
+[![Slack](https://img.shields.io/badge/slack-standardnotes-CC2B5E.svg?style=flat&logo=slack)](https://standardnotes.org/slack)
+[![GitHub Stars](https://img.shields.io/github/stars/standardnotes/secure-spreadsheets?style=social)](https://github.com/standardnotes/secure-spreadsheets)
+
+</div>
+
+## Introduction
+
+Secure Spreadsheets is a [derived editor](https://standardnotes.org/help/77/what-are-editors) for [Standard Notes](https://standardnotes.org), a free, [open-source](https://standardnotes.org/knowledge/5/what-is-free-and-open-source-software), and [end-to-end encrypted](https://standardnotes.org/knowledge/2/what-is-end-to-end-encryption) notes app. It is derived from https://github.com/telerik/kendo-ui-core.
+
+A demo of the Secure Spreadsheets editor is available at https://standardnotes.org/demo. To use Secure Spreadsheets with Standard Notes, please sign up for [Standard Notes Extended](https://standardnotes.org/extensions) and install it by following the instructions described [here](https://standardnotes.org/help/29/how-do-i-install-extensions-once-i-ve-signed-up-for-extended).
+
+## Development
+
+1. Run `npm install` to install the dependencies.
+2. Run `npm run start` to start the development server. Then visit http://localhost:8080/.
+3. Run `npm run build` to build the editor with Webpack.