doc: add reference in README.md

Author: dj8yf0μl

README.md

@@ -99,6 +99,12 @@ enum A {
 }
 ```
 
+## Advanced examples 
+
+Some of the less trivial examples are present in [examples](./borsh/examples) folder:
+
+- [implementing `BorshSerialize`/`BorshDeserialize` for third-party `serde_json::Value`](./borsh/examples/serde_json_value.rs)
+
 ## Testing
 
 Integration tests should generally be preferred to unit ones. Root module of integration tests of `borsh` crate is [linked](./borsh/tests/tests.rs) here.

borsh/examples/serde_json_value.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 
 use borsh::{BorshDeserialize, BorshSerialize};
 
-od serde_json_value {
+mod serde_json_value {
     pub use de::deserialize_value;
     pub use ser::serialize_value;
     mod ser {
@@ -246,6 +246,7 @@ struct SerdeJsonAsField {
 }
 
 fn main() {
+    // original code is from https://github.com/near/borsh-rs/pull/312
     let original = serde_json::json!({
         "null": null,
         "true": true,