Update the readme file to look almost identical to the nexmo-spring-boot-starter README file.

TODO

• Copy and paste the contents of the Nexmo Spring Boot Starter README file into the README for this repo.

• Make sure all references to Nexmo are changed to Vonage. Ex: nexmo.creds.api-key=your-api-key from the Configuration section should be changed to vonage.creds.api-key=your-api-key