Add FlexBuffers to Serde ecosystem list

Author: dtolnay

_src/README.md

@@ -66,6 +66,8 @@ Serde by the community.
 - [S-expressions], the textual representation of code and data used by the Lisp
   language family.
 - [D-Bus]'s binary wire format.
+- [FlexBuffers], the schemaless cousin of Google's FlatBuffers zero-copy
+  serialization format.
 
 [JSON]: https://github.com/serde-rs/json
 [Bincode]: https://github.com/TyOverby/bincode
@@ -86,6 +88,7 @@ Serde by the community.
 [AWS Parameter Store]: https://docs.aws.amazon.com/systems-manager/latest/userguide/systems-manager-paramstore.html
 [S-expressions]: https://github.com/rotty/lexpr-rs
 [D-Bus]: https://docs.rs/zvariant
+[FlexBuffers]: https://github.com/google/flatbuffers/tree/master/rust/flexbuffers
 
 ### Data structures