adds relevant badges to README.md (aws#416)

Updates README with relevant badges.

README.md

@@ -1,4 +1,4 @@
-![Build Status](https://codebuild.us-west-2.amazonaws.com/badges?uuid=eyJlbmNyeXB0ZWREYXRhIjoib1lGQ3N6RFJsalI5a3BPcXB3Rytaak9kYVh1ZW1lZExPNjgzaU9Udng3VE5OL1I3czIwcVhkMUlUeG91ajBVaWRYcVVJSEVQcmZwTWVyT1p5MGszbnA4PSIsIml2UGFyYW1ldGVyU3BlYyI6IkhrZ1VMN20zRmtYY1BrR0wiLCJtYXRlcmlhbFNldFNlcmlhbCI6MX0%3D&branch=master) [![Documentation](https://godoc.org/github.com/aws/aws-sdk-go-v2?status.svg)](https://godoc.org/github.com/aws/aws-sdk-go-v2)
+![Build Status](https://codebuild.us-west-2.amazonaws.com/badges?uuid=eyJlbmNyeXB0ZWREYXRhIjoib1lGQ3N6RFJsalI5a3BPcXB3Rytaak9kYVh1ZW1lZExPNjgzaU9Udng3VE5OL1I3czIwcVhkMUlUeG91ajBVaWRYcVVJSEVQcmZwTWVyT1p5MGszbnA4PSIsIml2UGFyYW1ldGVyU3BlYyI6IkhrZ1VMN20zRmtYY1BrR0wiLCJtYXRlcmlhbFNldFNlcmlhbCI6MX0%3D&branch=master) [![API Reference](https://img.shields.io/badge/api-reference-blue.svg)](https://docs.aws.amazon.com/sdk-for-go/v2/api) [![Join the chat at https://gitter.im/aws/aws-sdk-go](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/aws/aws-sdk-go-v2?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![Apache V2 License](https://img.shields.io/badge/license-Apache%20V2-blue.svg)](https://github.com/aws/aws-sdk-go/blob/master/LICENSE.txt)
 
 # AWS SDK for Go v2