Add note about rbenv for rvm .versions.conf local override

Author: jhart-r7

.gitignore

@@ -8,8 +8,10 @@ Gemfile.local.lock
 # RVM control file, keep this to avoid backdooring Metasploit
 .rvmrc
 # Allow for a local choice of (unsupported / semi-supported) ruby versions
-# See PR #4136 for usage, but example usage:
+# See PR #4136 for usage, but example usage for rvm:
 #   rvm --create --versions-conf use 2.1.4@metasploit-framework
+# Because rbenv doesn't use .versions.conf, to achieve this same functionality, run:
+#   rbenv shell 2.1.4
 .versions.conf
 # YARD cache directory
 .yardoc