Update documentation to explain why config:;standard() doesn't work with serde

[Icelk]

When I updated from beta-2 to beta-3, serialization and deserialization began to fail.
I get the error SkipFixedArrayLengthNotSupported.

See Icelk/agde#3 for the CI error. I'm not using any slices, just Vec.

This however works when serializing and deserializing using the standard methods and bincode::serde::Compat.

[VictorKoenders]

It turned out there was an issue with serde and not being able to encode fixed sized arrays. bincode::config::standard() sets skip_fixed_array_length by default. Because serde does not tell us that a fixed size array is being (de)serialized, and we simply get a slice, there was no way to detect this.

Because of this, we had to hard bail any bincode::serde::* function if .skip_fixed_array_length was set. If you use bincode::config::standard().write_fixed_array_length() it should all world.

[Icelk]

It now works as expected. Thanks for the swift reply and this library!

[VictorKoenders]

Gonna keep this open as a tracking issue to make the documentation more clear.

We should add a comment like my response on the following places:

• EncodeError and DecodeError. This is probably (hopefully) the place people look first.
• config::standard(). as this is the place where people need to add .write_fixed_array_length()
• config module as this is the place with the most explanation on config
• Configuration.skip_fixed_array_length already has it but Configuration.write_fixed_array_length doesn't

[VictorKoenders]

We decided to make .write_fixed_array_header the default when using config::standard() so it would work better with serde

#509