Added shrinkwrap command to docs. Updated docs to reflect latest version

[rmartin]

I ran into an issue when installing this module until I realized that ghost uses Shrinkwrap for it's modules. It looks like others in the community faced similar issues. https://stackoverflow.com/questions/32979024/node-js-heroku-ghost-is-unable-to-start-due-to-missing-dependencies-cannot-find/33003491#33003491

This updates the documentation to include the npm shrinkwrap command in the documentation to ensure that it updates the dependencies so that it can be installed.

I also updated the references to 0.6.0 to read 'or later' to reflect that it works with any version greater than 0.6.0.