docs(readme): update (absolute) link to CONTRIBUTING.md (#17)

link to contributing.md changed to absolute

Reason for this - is website https://serverless-api.twilio-labs.com/  that generated by this readme
doesn't actually have CONTRIBUTING.md

Author: butuzov

packages/serverless-api/README.md

@@ -141,7 +141,7 @@ const utils = require('@twilio-labs/serverless-api/dist/utils');
 
 ## Contributing
 
-This project welcomes contributions from the community. Please see the [`CONTRIBUTING.md`](CONTRIBUTING.md) file for more details.
+This project welcomes contributions from the community. Please see the [`CONTRIBUTING.md`](https://github.com/twilio-labs/serverless-api/blob/master/CONTRIBUTING.md) file for more details.
 
 ### Code of Conduct