README.md directs users to import using gopkg.in

Update to project README.md file recommending that users import this project using gopkg.in style import statements for both V1 and V2 of this library.

This is because V2 was released prior to the proposal to add package version support to the Go toolchain, which requires library authors to leave V1 code in the top level directory of the repository, and create a v2 directory for V2 of the library.