Link to compatibility matrix from Readme and docs landing page

[emanuil-tolev]

Expected Behavior

A new Curator user should be able to tell that Curator does not track the wider stack in terms of versions, and where to find the compatibility matrix. https://www.elastic.co/guide/en/elasticsearch/client/curator/5.5/version-compatibility.html

Actual Behavior

The readme on https://github.com/elastic/curator doesn't mention the matrix atm. It's easy to see "Build 5.6.0" and assume it just doesn't support 6 (someone on IRC seems to have, as did I).

Suggestion

Edit the readme, inserting a link to the matrix close to the Build Passing: version num indicator.

On the docs page https://www.elastic.co/guide/en/elasticsearch/client/curator/5.6/index.html this is a harder problem. I am very used to switching to 6.x for ES docs, so when I saw that dropdown and observed a lack of "6", I took that as further indication that Curator doesn't support ES 6.x. Not sure how to mitigate that - could enter a bit of text just above the docs table of contents, but it's quite clunky to say "this dropdown is for the Curator version, not the Stack version, despite looking the same and being hosted on the same documentation site". Hopefully sth on the github Readme will mitigate.

This should be pretty simple, I'll PR it when not up against another deadline.