update(docs): mention serde support

Signed-off-by: Grzegorz Nosek <[email protected]>

falco_event/README.md

@@ -128,3 +128,16 @@ you're not interested in.
 
 There is a trait ([events::EventToBytes]) that writes a serialized form of an event to a writer
 (i.e. a type that implements [std::io::Write], for example `Vec<u8>`).
+
+## Serialization and deserialization with [`serde`]
+
+Once you enable the `serde` feature of this crate, the following types gain serialization and deserialization
+support:
+
+* all the `PT_*` field types (many are Rust stdlib built-ins and support (de)serialization regardless)
+* all the [`event::types`] events (see the module documentation for details regarding owned/borrowed types)
+* `Event<AnyEvent>` (but *not* other `Event<T>` types, to avoid ambiguous serialization results and subsequent
+  deserialization issues)
+
+**Note**: you can serialize an event wrapping either the [borrowed](`event::types::AnyEvent`) or
+[owned](`event::types::owned::AnyEvent`) variant of `AnyEvent`, but can only deserialize to the owned variant.