Add docs for utility functions

[alulsh]

Lambda-cfn exposes two utility functions - splitOnComma and message - in https://github.com/mapbox/lambda-cfn/blob/14015e33f2ca0ec87bffa1866d5f61ab0f9d0ba6/index.js#L4-L5 that are useful when working with lambda-cfn based projects such as Patrol. For more context, you can see both of these functions being used in the function.js file for the cloudTrail rule in /patrol-rules-aws.

We should write brief docs on these functions in this repo, including why they might be useful and how to use them. Unless you're closely looking at the index.js file you wouldn't know about these functions, and it's not clear from the message() function's name what it does.

/cc @zmully @ianshward