Added the "Compatibility" chapter to the READMEs

In order to make the support for different Elasticsearch versions more clear,
a special "Compatibility" chapter has been added to the main README and
individual gem READMEs.

The versioning scheme follows the scheme for the Elasticsearch Ruby client,
ie. the version number matches the supported major version of Elasticsearch.

See:

* elastic/elasticsearch-ruby@3bdb385

Author: karmi

README.md

@@ -16,15 +16,27 @@ This repository contains various Ruby and Rails integrations for [Elasticsearch]
 Elasticsearch client and Ruby API is provided by the
 **[elasticsearch-ruby](https://github.com/elasticsearch/elasticsearch-ruby)** project.
 
-## Installation
+## Compatibility
 
 The libraries are compatible with Ruby 1.9.3 and higher.
 
-Install the `elasticsearch-model` and/or `elasticsearch-rails` package from
-[Rubygems](https://rubygems.org/gems/elasticsearch):
+The version numbers follow the Elasticsearch major versions, and the `master` branch
+is compatible with the Elasticsearch `master` branch, therefore, with the next major version.
+
+| Rubygem       |   | Elasticsearch |
+|:-------------:|:-:| :-----------: |
+| 0.1           | → | 1.x           |
+| 2.x           | → | 2.x           |
+| 5.x           | → | 5.x           |
+| master        | → | master        |
+
+## Installation
+
+Install each library from [Rubygems](https://rubygems.org/gems/elasticsearch):
 
 ```ruby
-gem install elasticsearch-model elasticsearch-rails
+gem install elasticsearch-model
+gem install elasticsearch-rails
 ```
 
 To use an unreleased version, either add it to your `Gemfile` for [Bundler](http://gembundler.com):

elasticsearch-model/README.md

@@ -1,13 +1,25 @@
 # Elasticsearch::Model
 
 The `elasticsearch-model` library builds on top of the
-the [`elasticsearch`](https://github.com/elasticsearch/elasticsearch-ruby) library.
+the [`elasticsearch`](https://github.com/elastic/elasticsearch-ruby) library.
 
 It aims to simplify integration of Ruby classes ("models"), commonly found
 e.g. in [Ruby on Rails](http://rubyonrails.org) applications, with the
 [Elasticsearch](http://www.elasticsearch.org) search and analytics engine.
 
-The library is compatible with Ruby 1.9.3 and higher.
+## Compatibility
+
+This library is compatible with Ruby 1.9.3 and higher.
+
+The library version numbers follow the Elasticsearch major versions, and the `master` branch
+is compatible with the Elasticsearch `master` branch, therefore, with the next major version.
+
+| Rubygem       |   | Elasticsearch |
+|:-------------:|:-:| :-----------: |
+| 0.1           | → | 1.x           |
+| 2.x           | → | 2.x           |
+| 5.x           | → | 5.x           |
+| master        | → | master        |
 
 ## Installation
 

elasticsearch-persistence/README.md

@@ -2,7 +2,19 @@
 
 Persistence layer for Ruby domain objects in Elasticsearch, using the Repository and ActiveRecord patterns.
 
-The library is compatible with Ruby 1.9.3 (or higher) and Elasticsearch 1.0 (or higher).
+## Compatibility
+
+This library is compatible with Ruby 1.9.3 and higher.
+
+The library version numbers follow the Elasticsearch major versions, and the `master` branch
+is compatible with the Elasticsearch `master` branch, therefore, with the next major version.
+
+| Rubygem       |   | Elasticsearch |
+|:-------------:|:-:| :-----------: |
+| 0.1           | → | 1.x           |
+| 2.x           | → | 2.x           |
+| 5.x           | → | 5.x           |
+| master        | → | master        |
 
 ## Installation
 

elasticsearch-rails/README.md

@@ -1,10 +1,22 @@
 # Elasticsearch::Rails
 
-The `elasticsearch-rails` library is a companion for
-the [`elasticsearch-model`](https://github.com/elasticsearch/elasticsearch-rails/tree/master/elasticsearch-model)
+The `elasticsearch-rails` library is a companion for the
+the [`elasticsearch-model`](https://github.com/elastic/elasticsearch-rails/tree/master/elasticsearch-model)
 library, providing features suitable for Ruby on Rails applications.
 
-The library is compatible with Ruby 1.9.3 and higher.
+## Compatibility
+
+This library is compatible with Ruby 1.9.3 and higher.
+
+The library version numbers follow the Elasticsearch major versions, and the `master` branch
+is compatible with the Elasticsearch `master` branch, therefore, with the next major version.
+
+| Rubygem       |   | Elasticsearch |
+|:-------------:|:-:| :-----------: |
+| 0.1           | → | 1.x           |
+| 2.x           | → | 2.x           |
+| 5.x           | → | 5.x           |
+| master        | → | master        |
 
 ## Installation