Create README.md

README.md

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+[![Build Status](https://travis-ci.org/bitrinjani/castable.svg?branch=master?i=2)](https://travis-ci.org/bitrinjani/castable) [![Coverage Status](https://coveralls.io/repos/github/bitrinjani/castable/badge.svg?branch=master&i=2)](https://coveralls.io/github/bitrinjani/castable?branch=master) [![npm version](https://badge.fury.io/js/%40bitr%2Fcastable.svg)](https://badge.fury.io/js/%40bitr%2Fcastable)
+
+# Castable TypeScript Library
+Castable sanitizes dirty external data by casting all properties at run time to the types specified at compile time.
+
+# Why do you need this library?
+A lot of web services return numbers **with double-quotes**. If you convert them by JSON.stringify, the double-quoted numbers will be string type!!
+
+```JavaScript
+const serverResponse = `{
+　　"name": "Milk", 
+　　"price": "200", 
+　　"tax": "10", 
+}`;
+const product = JSON.parse(serverResponse);
+const sum = product.price + product.tax;
+console.log(`sum: ${sum}`); // "200" + "10" = "20010"⛔️
+```
+
+TypeScript type annotation can help it? No, TypeScript cannot check such run-time type mismatch. You will get the exactly same result even type annotation is perfect.
+
+That's why I've made this library. Castable can convert those types at run time. All fields will be converted to the annotated types.
+
+```TypeScript
+import { cast, Castable } from 'castable';
+
+class Product extends Castable { 
+  @cast name: string;
+  @cast price: number;
+  @cast tax: number;
+}
+
+const serverResponse = `{"name": "Milk", "price": "200", "tax": "10"}`;
+const product = new Product(JSON.parse(serverResponse));
+const sum = product.price + product.tax;
+console.log(`sum: ${sum}`); // 200 + 10 = 210👍
+```
+
+This library also can convert:
+- Nested type
+- Array<T>
+- Multi-dementional Array<T> 
+
+# Install
+
+```bash
+npm install @bitr/castable
+```
+
+# Usage
+1. Extend Castable
+2. Add `@cast` decorator to primitive type field (string, number, boolean)
+3. Add `@cast(Date)` decorator to Date type field
+4. Add `@cast @element(T)` to Array<T> type field
+4. Add `@cast` decorator to nested type
+5. Do same to all nested types
+
+