Fix gem dependencies and add brief README

With a hat tip to @gpxl whose code enabled me to get the marketo gem working again - thanks!

On trying to get this going I found it wasn't working because savon has updated. The marketo_gem code doesn't seem work with the latest version of savon.

Quick solution, as @gpxl has already found, is to replace >= 0.8.3 with ~> 1.2.0 in the gemspec.

Another thing that slowed me down was finding some documentation. So I thought I'd add a tiny README.