Merge pull request #540 from duykhoa/master

docs: add installation step.

Author: djanowski

README.md

@@ -23,6 +23,20 @@ most important changes, as well as a full list of changes.
 
 ## Getting started
 
+To install **redis-rb**, run the following command:
+
+```
+  gem install redis
+```
+
+Or if you are using **bundler**, add 
+
+```
+  gem 'redis', '~>3.2'
+```
+
+to your `Gemfile`, and run `bundle install`
+
 As of version 2.0 this client only targets Redis version 2.0 and higher.
 You can use an older version of this client if you need to interface
 with a Redis instance older than 2.0, but this is no longer supported.