chore(v1): [#1] update README with installation and usage

README.md

@@ -1,2 +1,53 @@
 # aws-cdk-billing-alarm
 A CDK construct that sets up email notification for when you exceed a given AWS estimated charges amount.
+
+Create this construct in a billing stack **with only a few lines**. This construct is an implementation of the manual
+setup described on [AWS Estimated Charges Monitoring](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/gs_monitor_estimated_charges_with_cloudwatch.html#gs_creating_billing_alarm).
+Please read carefully and perform the entire setup process.
+
+## Get Started
+
+### Pre-Requisites
+
+> **IMPORTANT!** Only complete **_Step 1: Enable Billing Alerts_** of the following documentation link. This construct will take
+care of creating the rest of the resources for you.
+
+You must first enable billing alerts from the AWS Console as per [documentation](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/gs_monitor_estimated_charges_with_cloudwatch.html#gs_turning_on_billing_metrics).
+
+Billing alerts will allow your AWS account to start collecting billing metrics (`EstimatedCharges`) on a periodic 6-hour basis.
+
+### Installation and Usage
+#### Typescript
+
+```console
+npm install --save aws-cdk-billing-alarm
+```
+
+```typescript
+import * as cdk from '@aws-cdk/core';
+import { BillingAlarm } from 'aws-cdk-billing-alarm';
+
+export class CdkStack extends cdk.Stack {
+  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
+    super(scope, id, props);
+
+    // Create an alarm that emails admin@example.com
+    // if estimated charges exceed 50 USD
+    new BillingAlarm(stack, 'AWSAccountBillingAlarm', {
+      monthlyThreshold: 50,
+      email: 'admin@example.com',
+    });
+  }
+}
+```
+
+### Post-Deployment
+
+Confirm the subscription to the newly created topic for the email you specified as endpoint in `BillingAlarmProps`.
+You can do so by clicking on the `SubscribeURL` of the JSON email you received.
+> **Note**: If you did not receive the email, you can fire a **Request confirmation** for the subscription from the AWS SNS Console.
+   
+
+## Limitations
+
+- [USD currency](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/monitor_estimated_charges_with_cloudwatch.html#creating_billing_alarm_with_wizard)