Added a list of software that's required to run builds.

[twiest]

It's sometimes difficult to know what packages need to be installed in order for the standard go-lang build tools to work properly.

Also, sometimes the errors that are generated from missing packages aren't clear on what's needed to fix them.

This represents the minimum set of packages in order to get a go build, rpm build and image build working.

[stevekuznetsov]

Can we use the os::util::ensure libraries to make the error messages clearer?

[twiest]

@stevekuznetsov the 3 functions I see in os::util::ensure:: are system_binary_exists, gopath_binary_exists and built_binary_exists.

Were you thinking we should add something like system_package_exists ?

[stevekuznetsov]

We could probably just look for system_binary -- go, docker, git, whatever -- that would work well in addition to this README to help fix those bad error messages.