TASK: add README hint that the 4.x branch still supports ES 1 and 2

I've personally been thinking that we have dropped support for legacy versions; and it's a lot harder
to find the old version's configuration still. That's why I added a note to point to the 4.x branch.

Author: skurfuerst

README.md

@@ -29,6 +29,8 @@ Finally, run `./flow nodeindex:build`, and add the search plugin to your page. I
 
 ## Elastic version support
 
+**HINT: the Master of this package only supports modern versions of ES. If you need ES 1.x or 2.x support, please [see the 4.x branch of this repository](https://github.com/Flowpack/Flowpack.ElasticSearch.ContentRepositoryAdaptor/tree/4.0#elastic-version-support).**
+
 You can switch the Elastic driver by editing ```Settings.yaml```
 (```Flowpack.ElasticSearch.ContentRepositoryAdaptor.driver.version```) with the following value: