Add documentation for `Plug.Conn.Query.encode/2` for list of maps

[gaiabeatrice]

I had been stuck on encoding list of maps (of different size than 1), and I found https://github.com/elixir-plug/plug/issues/428#issuecomment-235673751 very useful. So I thought about adding it to the documentation.

Hopefully this can be helpful to somebody else in the future 😅

[whatyouhide]

Thanks @gaiabeatrice! 💟