Added "--package" argument to make-distribution.sh

[jmschrack]

"--package" creates a distribution without building.
Added the above info to the README.md.

[AmplabJenkins]

Can one of the admins verify this patch?

[pwendell]

Hey @jmschrack - I'm not sure we want to allow packaging without building in this script. This allows for some confusing outcomes, such as a user compiles Spark for one version, then they call make-distribution with different maven flags, and the naming of the distribution and notes are inconsistent with the actual binaries. Since make-distribution takes identical flags to the build, I'd rather just force people to build and package atomically. So I'd prefer to close this issue for now and feel free to re-open if you'd like to discuss further.