Add gem install line to README since the gem name does not match the repo name

See #207

Sorry about that @maia

Author: oreoshake

README.md

@@ -1,4 +1,5 @@
-# SecureHeaders [![Build Status](https://travis-ci.org/twitter/secureheaders.png?branch=master)](http://travis-ci.org/twitter/secureheaders) [![Code Climate](https://codeclimate.com/github/twitter/secureheaders.png)](https://codeclimate.com/github/twitter/secureheaders) [![Coverage Status](https://coveralls.io/repos/twitter/secureheaders/badge.png)](https://coveralls.io/r/twitter/secureheaders)
+# Secure Headers [![Build Status](https://travis-ci.org/twitter/secureheaders.png?branch=master)](http://travis-ci.org/twitter/secureheaders) [![Code Climate](https://codeclimate.com/github/twitter/secureheaders.png)](https://codeclimate.com/github/twitter/secureheaders) [![Coverage Status](https://coveralls.io/repos/twitter/secureheaders/badge.png)](https://coveralls.io/r/twitter/secureheaders)
+
 
 **The 3.x branch was recently merged**. See the [upgrading to 3.x doc](upgrading-to-3-0.md) for instructions on how to upgrade including the differences and benefits of using the 3.x branch.
 
@@ -16,6 +17,10 @@ The gem will automatically apply several headers that are related to security.
 
 `secure_headers` is a library with a global config, per request overrides, and rack milddleware that enables you customize your application settings.
 
+## Use
+
+`gem install secure_headers`
+
 ## Configuration
 
 If you do not supply a `default` configuration, exceptions will be raised. If you would like to use a default configuration (which is fairly locked down), just call `SecureHeaders::Configuration.default` without any arguments or block.