(maint) Add semantic versioning info to README
(maint) Add semantic versioning info to README

This patch adds semantic versioning information to the README of this
module.  This information is missing and unclear without this patch.
This should help clarify the support matrix for the Standard Library as
it relates to Puppet Enterprise released versions.

fuel-infra/puppetlabs-stdlib,puppetlabs/puppetlabs-stdlib,jbondpdx/puppetlabs-stdlib,DavidS/puppetlabs-stdlib,HelenCampbell/puppetlabs-stdlib,yadavnikhil/puppetlabs-stdlib