docs: add README for @notesnook/core

packages/core/README.md

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+<h1 align="center">Notesnook Core</h1>
+<h3 align="center">The shared core for building Notesnook on any platform.</h3>
+<p align="center">
+<a href="#developer-guide">Developer guide</a> | <a href="#build-instructions">How to build?</a> | <a href="#running-the-tests">Run the tests</a>
+</p>
+
+## Build instructions
+
+**Before you start it is recommended that you read [the contributing guidelines](/CONTRIBUTING.md).**
+
+### Setting up the development environment
+
+Requirements:
+
+1. [Node.js](https://nodejs.org/en/download/)
+2. [git](https://git-scm.com/downloads)
+3. NPM (not yarn or pnpm)
+
+Before you can do anything, you'll need to [install Node.js](https://nodejs.org/en/download/) on your system.
+
+Once you have completed the setup, the first step is to `clone` the monorepo:
+
+```bash
+git clone https://github.com/streetwriters/notesnook.git
+
+# change directory
+cd notesnook
+```
+
+Once you are inside the `./notesnook` directory, run the preparation step:
+
+```bash
+# this might take a while to complete
+npm install
+```
+
+And that's it. You can run the tests to make sure everything is working as it should:
+
+```bash
+npm run test:core
+```
+
+## Developer guide
+
+### The tech stack
+
+We try to keep the stack as lean as possible
+
+1. Javascript
+2. Jest: Testing framework
+
+### Running the tests
+
+When you are done making the required changes, you need to run the tests. We use Jest as the testing framework & the tests can be run with a single command:
+
+```bash
+npm run test:core
+```