Add indifferent matching configuration option to the README.md

Author: sa73917

README.md

@@ -48,6 +48,17 @@ Available matchers:
 * `expect(document).to have_jsonapi_object`
 * `expect(document).to have_jsonapi_object('version' => '1.0')`
 
+### Indifferent Matching
+```ruby
+# spec/spec_helpers.rb
+
+RSpec.configure do |config|
+  # ...
+  config.allow_symbolized_jsonapi = true
+end
+```
+The configuration above allows the RSpec matchers to work with a symbolized jsonapi source document.
+
 ## Advanced examples
 
 Checking for an included resource: