This is really a nitpick, but I would add backticks and blank lines to make it a bit more readable, like this:

~~~markdown
Add a `:serializer` option for `config.session_store :cookie_store`. This
changes default serializer when using `:cookie_store` to
`ActionDispatch::Session::MarshalSerializer` which is a wrapper around Marshal.

It is also possible to pass:

* `:json_serializer` which is secure wrapper on JSON using parse / generate API.
* any other Symbol or String like `:my_custom_serializer` which will be
camelized and constantized in `ActionDispatch::Session namespace`.
* serializer object with load / dump methods defined.

*Łukasz Sarnacki*
~~~

What do you think ? Apart from that, it looks great! :+1: