(MCO-386) MCO README.md should express semver

The puppet readme (https://github.com/puppetlabs/puppet/blob/master/README.md)
explains the maintenance we have for versions of puppet and that it is
semantically versioned. We recently encountered an issue with Facter where
users were surprised that we did not ship a security release for a previous
major version of facter. The previous assumption may have been that the puppet
README was sufficient for all of our FOSS projects, but this is probably not
the case. We should update the README.md in mcollective with the same verbage as
puppet so our intended release lifecycle is clear.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
This isn't how we've been applying semver, or how I expect us to.

If a security bug is found then we'd fix it in our stable branch, and then we'd cut a release for it.  The version number at this point would follow semver semantics considering our interfaces and apis, so we may end up with a progression 2.5.2 -> 2.6.0 if enough configuration or exposed apis are changed.

For an example of this consider the changes to around the connector reliability in 2.5.0.  2.5.0 isn't that much further on from 2.4.1, but it did introduce a new exception class, which is a new api feature.