Update dependencies documentation to include (probably) required serde in time crate

[0ba100]

This is probably only required when using the time feature. I came across the following error without it:

error[E0277]: the trait bound `PrimitiveDateTime: Serialize` is not satisfied
    --> /workspace/target/debug/build/db-d6aaecb6f4eed2cf/out/cornucopia.rs:7:102
     |
7    | ...utures; use cornucopia_async::GenericClient;#[derive(serde::Serialize, Debug, Clone,...
     |                                                         ^^^^^^^^^^^^^^^^ the trait `Serialize` is not implemented for `PrimitiveDateTime`
8    | ...n : String,pub start_time : time::PrimitiveDateTime,}pub struct EventBorrowed<'a> { ...
     |               --- required by a bound introduced by this call
     |
     = note: for local types consider adding `#[derive(serde::Serialize)]` to your `PrimitiveDateTime` type
     = note: for types from other crates check whether the crate offers a `serde` feature flag
     = help: the following other types implement trait `Serialize`:
               &'a T
               &'a mut T
               ()
               (T,)
               (T0, T1)
               (T0, T1, T2)
               (T0, T1, T2, T3)
               (T0, T1, T2, T3, T4)
             and 134 others
note: required by a bound in `events::_::_serde::ser::SerializeStruct::serialize_field`
    --> /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.210/src/ser/mod.rs:1867:21
     |
1865 |     fn serialize_field<T>(&mut self, key: &'static str, value: &T) -> Result<(), Self::...
     |        --------------- required by a bound in this associated function
1866 |     where
1867 |         T: ?Sized + Serialize;
     |                     ^^^^^^^^^ required by this bound in `SerializeStruct::serialize_field`
     = note: this error originates in the derive macro `serde::Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

it's only a small change but might help guide some folks.

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