Merge branch 'params-docs'

Closes #491, references #182

[ci skip]

README.md

@@ -65,6 +65,32 @@ stack and default adapter (see [Faraday::RackBuilder#initialize](https://github.
 response = Faraday.get 'http://sushi.com/nigiri/sake.json'
 ```
 
+### Changing how parameters are serialized
+
+Sometimes you need to send the same URL parameter multiple times with different
+values. This requires manually setting the parameter encoder and can be done on
+either per-connection or per-request basis.
+
+```ruby
+# per-connection setting
+conn = Faraday.new :params_encoder => Faraday::FlatParamsEncoder
+
+conn.get do |req|
+  # per-request setting:
+  # req.options.params_encoder = my_encoder
+  req.params['roll'] = ['california', 'philadelphia']
+end
+# GET 'http://sushi.com?roll=california&roll=philadelphia'
+```
+
+The value of Faraday `params_encoder` can be any object that responds to:
+
+* `encode(hash) #=> String`
+* `decode(string) #=> Hash`
+
+The encoder will affect both how query strings are processed and how POST bodies
+get serialized. The default encoder is Faraday::NestedParamsEncoder.
+
 ## Advanced middleware usage
 
 The order in which middleware is stacked is important. Like with Rack, the