Updating docs to use mainline as main branch

cr https://code.amazon.com/reviews/CR-39895130

CONTRIBUTING.md

@@ -11,7 +11,7 @@ an issue to discuss it.
 
 ### Making the request
 
-Development takes place against the `master` branch of this repository and pull
+Development takes place against the `mainline` branch of this repository and pull
 requests should be opened against that branch.
 
 ### Testing

README.md

 [![ReportCard][ReportCard-Image]][ReportCard-URL]
-[![Build Status](https://travis-ci.org/aws/amazon-ssm-agent.svg?branch=master)](https://travis-ci.org/aws/amazon-ssm-agent)
+[![Build Status](https://travis-ci.org/aws/amazon-ssm-agent.svg?branch=mainline)](https://travis-ci.org/aws/amazon-ssm-agent)
 
 # Amazon SSM Agent
 
@@ -137,7 +137,7 @@ The following targets are available. Each may be run with `make <target>`.
 
 ### Contributing
 
-Contributions and feedback are welcome! Proposals and Pull Requests will be considered and responded to. Please see the [CONTRIBUTING.md](https://github.com/aws/amazon-ssm-agent/blob/master/CONTRIBUTING.md) file for more information.
+Contributions and feedback are welcome! Proposals and Pull Requests will be considered and responded to. Please see the [CONTRIBUTING.md](https://github.com/aws/amazon-ssm-agent/blob/mainline/CONTRIBUTING.md) file for more information.
 
 Amazon Web Services does not currently provide support for modified copies of this software.