Added chapter about development to the main README

Author: karmi

README.md

@@ -87,6 +87,61 @@ Keep in mind, that for optimal performance, you should use a HTTP library which
 * [Documentation](http://rubydoc.info/gems/elasticsearch-extensions/)
 * [Test Suite](https://github.com/elasticsearch/elasticsearch-ruby/blob/master/elasticsearch-extensions/test)
 
+## Development
+
+To work on the code, clone and bootstrap the project first:
+
+```
+git clone https://github.com/elasticsearch/elasticsearch-ruby.git
+cd elasticsearch-ruby/
+rake setup
+rake bundle
+```
+
+This will clone the Elasticsearch repository into the project, and run `bundle install` in all subprojects.
+
+To run tests, you need to start a testing cluster on port 9250,
+or provide a different one in the `TEST_CLUSTER_PORT` environment variable.
+
+There's a Rake task to start the testing cluster:
+
+```
+rake test:cluster:start
+```
+
+You can configure the port, path to the startup script,
+number of nodes, and other settings with environment variables:
+
+```
+TEST_CLUSTER_COMMAND=./tmp/builds/elasticsearch-2.0.0-SNAPSHOT/bin/elasticsearch \
+TEST_CLUSTER_PORT=9250 \
+TEST_CLUSTER_NODES=2 \
+TEST_CLUSTER_NAME=my_cluster \
+TEST_CLUSTER_PARAMS='-Xms500m -Xmx500m -D es.index.store.type=niofs' \
+TEST_CLUSTER_TIMEOUT=120 \
+rake test:cluster:start
+```
+
+To run tests against unreleased Elasticsearch versions, you can use the `rake elasticsearch:build`
+Rake task to build Elasticsearch from the cloned source
+(use `rake elasticsearch:update` to update the repository):
+
+```
+rake elasticsearch:build
+```
+
+You can pass a branch name (tag, commit, ...) as the Rake task variable:
+
+```
+rake elasticsearch:build[origin/1.x]
+```
+
+To run all the tests in all the subprojects, use the Rake task:
+
+```
+time rake test:all
+```
+
 ## License
 
 This software is licensed under the Apache 2 license, quoted below.