Add params example and clarify argument type
Add params example and clarify argument type

Since you have to provide types for each item in the argument tuple of the http verb method (e.g., `get`, `post`), and the error message you get when you try to use, e.g., `ids: List[String]` along with `:: params("ids")` takes a bit of scrutiny to figure out what it wants, I thought an example with the proper type would be helpful to newcomers like me.

Side question: I initially thought "multi-value param" would simply mean 'all the query parameters with that name will be aggregated and their values supplied in a single `Seq[String]`'. But it turns out finch also splits `params`-style query parameters on commas, and filters out empty strings. Which is handy in my current app, but more magical than I would have expected, and I'm not sure how I could get the pre-processed values if I wanted. Empty strings can be significant; I know of some client-side routing libraries (e.g., Angular's ui-router) that don't serialize query parameters with `null` or `undefined` values into the URL at all, but _do_ serialize `""` values as, well, empty strings. Anyways, the question was, is this behavior documented somewhere else, like maybe upstream in Finagle?
rpless/finch,finagle/finch