Add apt-cacher-ng to stemcell builder README
Add apt-cacher-ng to stemcell builder README

Speeds up iterations while working on the stemcell builder.

[#56367234]

Signed-off-by: Jesse Zhang <4e375e814be137a4f94f9d2b2a5f4ca930cc7a5a@pivotallabs.com>

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh