docs: add getting started guide to Gitbook

Author: NoNameProvided

.gitbook.yaml

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+root: ./docs
+​structure:  
+  readme: pages/01-getting-started.md  
+  summary: SUMMARY.md​

docs/SUMMARY.md

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+# Table of contents
+
+- [Getting Started](pages/01-getting-started.md)

docs/pages/01-getting-started.md

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+# Getting Started
+
+The `class-transformer` package is a zero-dependency utility library helping you to quickly transform class instances to plain objects and vice-versa.
+It works well with the [`class-validator`][class-validator] library. The main features include:
+
+- conditionally transforming object properties
+- excluding specific properties from the transformed object
+- exposing properties under a different name on the transformed object
+- supports both NodeJS and browsers
+- fully three-shakable
+- zero external dependencies
+
+## Installation
+
+To start using class-transformer install the required packages via NPM:
+
+```bash
+npm install class-transformer reflect-metadata
+```
+
+Import the `reflect-metadata` package at the **first line** of your application:
+
+```ts
+import 'reflect-metadata';
+
+// Your other imports and initialization code
+// comes here after you imported the reflect-metadata package!
+```
+
+As the last step, you need to enable emitting decorator metadata in your Typescript config. Add these two lines to your `tsconfig.json` file under the `compilerOptions` key:
+
+```json
+"emitDecoratorMetadata": true,
+"experimentalDecorators": true,
+```
+
+Now you are ready to use class-transformer with Typescript!
+
+## Basic Usage
+
+The most basic usage is to transform a class to a plain object:
+
+```ts
+import { Expose, Exclude, classToPlain } from 'class-transformer';
+
+class User {
+  /**
+   * When transformed to plain the `_id` property will be remapped to `id`
+   * in the plain object.
+   */
+  @Expose({ name: 'id' })
+  private _id: string;
+
+  /**
+   * Expose the `name` property as it is in the plain object.
+   */
+  @Expose()
+  public name: string;
+
+  /**
+   * Exclude the `passwordHash` so it won't be included in the plain object.
+   */
+  @Exclude()
+  public passwordHash: string;
+}
+
+const user = getUserMagically();
+// contains: User { _id: '42', name: 'John Snow', passwordHash: '2f55ce082...' }
+
+const plain = classToPlain(user);
+// contains { id: '42', name: 'John Snow' }
+```
+
+[class-validator]: https://github.com/typestack/class-validator/