Touch up go mod instructions in install
Touch up go mod instructions in install

Add steps on how to use your own fork to vendor in code using go mod
to the install instructions.

Signed-off-by: TomSweeneyRedHat <tsweeney@redhat.com>

Closes: #1781
Approved by: rhatdan

nalind/buildah-1,projectatomic/buildah,nalind/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah