Update README buildpack testing and packaging instructions
Update README buildpack testing and packaging instructions

[#149958934]

Signed-off-by: Keaty Gross <ba27100af1ae477f2abedd28f15b1dd208b1f48a@pivotal.io>

cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack