Document Environment Versions in Readme

[trevorghess]

We're trying to emulate a legacy Chef environment, it is important we're consistent in the tooling versions we select in this process. We've elected to start with Chef Server 12.1.2, we should make sure we're using a close version of Chef Client and Chef Workstation (Likely a version of ChefDK). We need to make sure that the content we generate has not been pre-optimized by newer versions of our kit, or we may miss things we need to include in the upgrade tool.

@irvingpop can you share the earliest common version of Chef Client and ChefDK we've seen at customers?

[irvingpop]

Based on our current telemetry capabilities, this isn't going to be easy to determine. The closest direct sources of data might be Hosted Chef or downloads.chef.io, but I worry those will also be skewed compared to typical enterprise deploys.

I couple of data points I did dig up:

1. ChefDK 0.10.0 was the last release of 2015, it contained Chef 12.5.1
2. Chef Client older than 12.3 does not save its complete (expanded) run list to the chef server, making it much harder to calculate cookbook dependencies from these nodes. We've noted the same thing in knife-tidy