Update format documentation to include additional format verbs about matrix

[btracey]

Because of internal packages, you need to have your fork be in a gonum-like path on your filesystem.

This seems to have a good description: http://stackoverflow.com/questions/14323872/using-forked-package-import-in-go

[btracey]

It's a good idea to add such an example

But there is no simple makefile to install all of the repo's dependencies so my test fails.

Did you try go get -u -t?

[DavidGamba]

This is what I get:

go get -u -t
package github.com/davidgamba/matrix/mat64
        imports github.com/gonum/internal/asm: use of internal package not allowed
go test
package github.com/davidgamba/matrix/mat64
        imports github.com/gonum/internal/asm: use of internal package not allowed