Add Compatibility Matrix from README to docs

[joshuar]

Hey team,

Would we be able to add the compatibility matrix from here:

https://github.com/elastic/elasticsearch-net#compatibility-matrix

In the docs? Potentially before the Breaking Changes or on the intro page?

[russcam]

Good idea @joshuar; I'll need to implement table parsing in our asciidoc parser for this 😄

[Mpdreamz]

closing this in favor of new tracking issue: #2248