(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.

Author: Jeff McCune

README.markdown

@@ -14,11 +14,34 @@ This module is officially curated and provided by Puppet Labs.  The modules
 Puppet Labs writes and distributes will make heavy use of this standard
 library.
 
+# Versions #
+
+This module follows semver.org (v1.0.0) versioning guidelines.  The standard
+library module is released as part of [Puppet
+Enterprise](http://puppetlabs.com/puppet/puppet-enterprise/) and as a result
+older versions of Puppet Enterprise that Puppet Labs still supports will have
+bugfix maintenance branches periodically "merged up" into master.  The current
+list of integration branches are:
+
+ * v2.1.x (v2.1.1 released in PE 1.2, 1.2.1, 1.2.3, 1.2.4)
+ * v2.2.x (Never released as part of PE, only to the Forge)
+ * v2.3.x (Scheduled for next PE feature release)
+ * master (mainline development branch)
+
+The first Puppet Enterprise version including the stdlib module is Puppet
+Enterprise 1.2.
+
 # Compatibility #
 
-This module is designed to work with Puppet version 2.6 and later.  It may be
-forked if Puppet 2.7 specific features are added.  There are currently no plans
-for a Puppet 0.25 standard library module.
+## stdlib v2.1.x, v2.2.x ##
+
+v2.1.x and v2.2.x of this module are designed to work with Puppet versions
+2.6.x and 2.7.x.  There are currently no plans for a Puppet 0.25 standard
+library module.
+
+## stdlib v2.3.x ##
+
+While not yet released, the standard library may only work with Puppet 2.7.x.
 
 # Functions #
 
@@ -57,4 +80,3 @@ automatically.  This is a direct copy of R.I. Pienaar's custom facter fact
 located at:
 [https://github.com/ripienaar/facter-facts/tree/master/facts-dot-d](https://github.com/ripienaar/facter-facts/tree/master/facts-dot-d)
 
-