Add necessity of feature flag serde to api documentation for saveload

## Description
The [saveload documentation](https://specs.amethyst.rs/docs/tutorials/13_saveload.html?highlight=serde#saveload) states:
`saveload is a module that provides mechanisms to serialize and deserialize a World, it makes use of the popular [serde](https://docs.rs/serde) library and requires the feature flag serde to be enabled for specs in your Cargo.toml file.`

The [API documentation](https://docs.rs/specs/latest/specs/saveload/index.html) does not mention this fact, but it was at least for me the first google search hit when I got `unresolved import specs::saveload`.
Maybe it would even be worth it to add the information to the [Troubleshooting section](https://specs.amethyst.rs/docs/tutorials/14_troubleshooting.html)

## Motivation
I had to search quite a long time to find that information. I assume I'm not the only person to search the API than to read the tutorial.

## Drawbacks
None

* Is it a breaking change? No
* Can it impact performance, learnability, etc? Yes