Add versioning to example readme (#7152)

plugins/inputs/EXAMPLE_README.md

@@ -4,6 +4,9 @@ The `example` plugin gathers metrics about example things.  This description
 explains at a high level what the plugin does and provides links to where
 additional information can be found.
 
+Telegraf minimum version: Telegraf x.x  
+Plugin minimum tested version: x.x
+
 ### Configuration
 
 This section contains the default TOML to configure the plugin.  You can