[DOCS] Added a table with 0.90/1.0 compatibility and corresponding instructions

docs/ruby/index.asciidoc

@@ -7,23 +7,47 @@ to Elasticsearch clusters, Ruby interface to the REST API, and more.
 
 See the full documentation at http://github.com/elasticsearch/elasticsearch-ruby.
 
+=== Elasticsearch Version Compatibility
+
+The Ruby libraries are compatible with both Elasticsearch 0.90.x and 1.0.x versions,
+but you have to install a matching gem version:
+
+[cols="<,<",options="header",]
+|=========================================
+| Elasticsearch version | Ruby gem version
+| 0.90.x                | 0.4.x
+| 1.0.x                 | 1.x
+|=========================================
 
 === Installation
 
-Install the Ruby gem with:
+Install the Ruby gem for Elasticsearch *1.x*:
 
 [source,sh]
 ------------------------------------
 gem install elasticsearch
 ------------------------------------
 
-or add it do your Gemfile:
+...or add it do your Gemfile:
 
 [source,ruby]
 ------------------------------------
 gem 'elasticsearch'
 ------------------------------------
 
+Install the Ruby gem for Elasticsearch *0.90.x*:
+
+[source,sh]
+------------------------------------
+gem install elasticsearch -v 0.4.8
+------------------------------------
+
+...or add it do your Gemfile:
+
+[source,ruby]
+------------------------------------
+gem 'elasticsearch', '~> 0.4'
+------------------------------------
 
 === Example Usage