Add content to readme

Fixes #70

README.md

@@ -1,3 +1,23 @@
 # SwiftBeanCountModel
 
 [![CI Status](https://github.com/Nef10/SwiftBeanCountModel/workflows/CI/badge.svg?event=push)](https://github.com/Nef10/SwiftBeanCountModel/actions?query=workflow%3A%22CI%22) [![Documentation percentage](https://nef10.github.io/SwiftBeanCountModel/badge.svg)](https://nef10.github.io/SwiftBeanCountModel/) [![License: MIT](https://img.shields.io/github/license/Nef10/SwiftBeanCountModel)](https://github.com/Nef10/SwiftBeanCountModel/blob/master/LICENSE) [![Latest version](https://img.shields.io/github/v/release/Nef10/SwiftBeanCountModel?label=SemVer&sort=semver)](https://github.com/Nef10/SwiftBeanCountModel/releases) ![platforms supported: linux | macOS | iOS | watchOS | tvOS](https://img.shields.io/badge/platform-linux%20%7C%20macOS%20%7C%20iOS%20%7C%20watchOS%20%7C%20tvOS-blue) ![SPM compatible](https://img.shields.io/badge/SPM-compatible-blue)
+
+### ***This project is part for SwiftBeanCount, please check out the main documentation [here](https://github.com/Nef10/SwiftBeanCount).***
+
+## What
+
+This is the core of SwiftBeanCount, the model and the logic of the ledger.
+
+## How
+
+Please check out the complete documentation of this library [here](https://nef10.github.io/SwiftBeanCountModel/).
+
+## Usage
+
+The library supports the Swift Package Manger, so simply add a dependency in your `Package.swift`:
+
+```
+.package(url: "https://github.com/Nef10/SwiftBeanCountModel.git", .exact("X.Y.Z")),
+```
+
+*Note: as per semantic versioning all versions changes < 1.0.0 can be breaking, so please use `.exact` for now*