Add API information and overview to README

Here's a quick and short README that goes through an overview (of why you want to use Express Map) and gives simple examples of the API.

Let me know if this needs any changes - thanks!

**TODO**:

- [x] Fix wording suggestions by @drewfish 
- [ ] Consider renaming package/API methods/parameters
- [ ] Better explain use case of `app.getRouteParams`