zbx_group vs zbx_groups in Readme

[aberends]

What are you seeing

In the README.md file the parameter zbx_group is mentioned. In the code this parameter is deprecated in favor of zbx_groups. But in the README.md only zbx_group is mentioned.

What behaviour did you expect instead

I would expect to see in the README.md that zbx_group is deprecated and mentioning of the newer way, i.e. zbx_groups.

Any additional information you'd like to impart

Also related to issue #445.

[igalic]

@aberends, do you think you could provide us with a patch here?