Compatibility info for Elastic language clients

Author: ezimuel

README.md

@@ -87,6 +87,13 @@ Version Matrix
  - If you are using a version older than 1.0, you must install the `0.4` Elasticsearch-PHP branch. Since ES 0.90.x and below is now EOL, the corresponding `0.4` branch will not receive any more development or bugfixes.  Please upgrade.
  - You should never use Elasticsearch-PHP Master branch, as it tracks Elasticsearch master and may contain incomplete features or breaks in backwards compatibility. Only use ES-PHP master if you are developing against ES master for some reason.
 
+Compatibility
+-------------
+
+Language clients are forward compatible; meaning that clients support communicating
+with greater minor versions of Elasticsearch. Elastic language clients are also backwards
+compatible with lesser supported minor Elasticsearch versions.
+
 Documentation
 --------------
 [Full documentation can be found here.](https://www.elastic.co/guide/en/elasticsearch/client/php-api/current/index.html)  Docs are stored within the repo under /docs/, so if you see a typo or problem, please submit a PR to fix it!