docs: add readme.md

readme.md

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+# aws-sns-message-attributes [![CircleCI](https://circleci.com/gh/shelfio/aws-sns-message-attributes/tree/master.svg?style=svg)](https://circleci.com/gh/shelfio/aws-sns-message-attributes/tree/master) ![](https://img.shields.io/badge/code_style-prettier-ff69b4.svg) [![npm (scoped)](https://img.shields.io/npm/v/@shelf/aws-sns-message-attributes.svg)](https://www.npmjs.com/package/@shelf/aws-sns-message-attributes)
+
+> Transform JSON into AWS SNS message attributes format. See [1](https://docs.aws.amazon.com/sns/latest/dg/sns-message-attributes.html), [2](https://docs.aws.amazon.com/sns/latest/dg/sns-subscription-filter-policies.html)
+
+## Install
+
+```
+$ yarn add @shelf/aws-sns-message-attributes
+```
+
+## Usage
+
+```js
+const {transformMessageAttributes} = require('@shelf/aws-sns-message-attributes');
+
+transformMessageAttributes({
+  string: 'hello',
+  array: ['hello', 'world'],
+  number: 1
+});
+
+/*
+{
+  string: {DataType: 'String', StringValue: 'hello'},
+  array: {DataType: 'String.Array', StringValue: '["hello","world"]'},
+  number: {DataType: 'Number', StringValue: 1}
+}
+ */
+```
+
+## License
+
+MIT © [Shelf](https://shelf.io)