README.md should state how to get packer validate to work with this plugin

[taliesins]

I think in the main README we should probably mention that you should use packer validate test.pkr.hcl -evaluate-datasources with this plugin. Even though its a Packer problem, make it easy for your community. This cost me time and frustration.

-evaluate-datasources

This is a good indication that other people are running into the issue
#3
#4
#40

[baizon]

Added your recommended note with: ce8f6b0