Update README.md
support tgz buildpacks
Merge pull request #13 from ryandotsmith/master

Support .tgz buildpack format
Make bin/release optional.
Merge pull request #14 from dpiddy/optional-release

Make bin/release optional.
pass the new envfile argument to compile
show users to use tags; encouraging use of version pinning
Merge pull request #22 from kennethreitz/patch-1

show users to use tags; encouraging use of version pinning
Merge pull request #21 from zeke/master

pass the new envfile argument to compile