add development instructions

[afeld]

Fixes #2.

/cc moovweb/gokogiri#95

[jcscottiii]

This project uses the vendor folder only supported by Go1.6+ by default (1.5 if you use some env variables). But let's just support 1.6

Your README needs to tell people to use 1.6

[afeld]

I happen to be using the vendor/ folder, but it would still work in older versions of Go with the old dependency structure, right?

[jcscottiii]

it would work if the person with Go version <1.6 runs go get personally on their computer. But otherwise, all the old toolchains only look in certain places for dependencies. Go1.6 adds the vendor folder for a new location.

[afeld]

D'oh, I get it now. Forgot that we were checking the dependencies into the repository. Updated!