docs: Add docs to explain the new option

Author: Étienne Lévesque

README.md

@@ -75,6 +75,13 @@ def view() do
 end
 ```
 
+### JSON library selection
+
+ExCSSModules needs to parse JSON files in order to read the mappings between the base classe names and the autogenerated
+ones. You can specify which JSON parsing library you wish to use through the `:json_library` config option. If your
+project uses Phoenix, it is recommended to use the same library for both Phoenix and ExCSSModules. ExCSSModules uses
+[Poison](https://hex.pm/packages/poison) by default.
+
 ### ExCell
 ExCSSModules works perfectly with [ExCell](https://github.com/DefactoSoftware/ex_cell). By adding the following to your `web.ex` definition for cells you can automatically add the stylesheet in the same directory as your ExCell Cell:
 ```ex