Document the known bug with "ensure => 'latest'" for jenkins::plugin

This should close voxpupuli#98, but ultimately be fixed in Duckworth

README.markdown

@@ -31,12 +31,18 @@ The names of the plugins can be found on the [update
 site](http://updates.jenkins-ci.org/download/plugins)
 
 #### Latest
+
 By default, the resource will install the latest plugin, i.e.:
 
     jenkins::plugin {
       "git" : ;
     }
 
+If you specify `version => 'latest'` in current releases of the module, the
+plugin will be downloaded and installed with *every* run of Puppet. This is a
+known issue and will be addressed in future releases. For now it is recommended
+that you pin plugin versions when using the `jenkins::plugin` type.
+
 #### By version
 If you need to peg a specific version, simply specify that as a string, i.e.: