Document page_size option. Closes #25

Author: excid3

README.md

@@ -76,7 +76,6 @@ Receipts.default_font = {
 }
 ```
 
-
 ### Options
 
 You can pass the following options to generate a PDF:
@@ -130,6 +129,15 @@ Here's an example of where each option is displayed.
 
 See [the Prawn docs](https://prawnpdf.org/api-docs/2.3.0/Prawn/Text.html#text-instance_method) for more information.
 
+
+##### Page Size
+
+You can specify a different page size by passing in the `page_size` keyword argument:
+
+```ruby
+receipt = Receipts::Receipt.new page_size: "A4"
+```
+
 ### Internationalization (I18n)
 
 You can use `I18n.t` when rendering your receipts to internationalize them.