Describe the enhancement:
Docs should indicate the version of ECS a module was written against

Describe a specific use case for the enhancement or feature:
From @leehinman:

"With the ever increasing number of modules that Filebeat (and other beats) support and the release of new versions of ECS we are running into an interesting problem. Right now we don't know what version of ECS a particular module was written against.

So if our users want to search for a specific ECS field, they can't tell if the module should support it. For example, a user might want to search for tls.protocol on data ingested with the Apache module, but that won't work because that module was written before ECS supported those fields. And as a maintainer I don't know what the gap is between when the module was written and current ECS fields."