chore: add example of folding a circom circuit and explain how to use a

custom `step_native` function

src/usage/frontend-circom.md

@@ -38,5 +38,50 @@ template Example () {
     ivc_output[0] <== temp1 * ivc_input[0] + temp2 + external_inputs[1];
 }
 
-component main {public [ivc_input]} = Example();
+component main {public [ivc_input]} = CubicCircuit();
 ```
+
+Once your `circom` circuit is ready, you can instantiate it with Sonobe. To do this, you will need the `struct CircomFCircuit`.
+
+```rust
+// we load our circom compiled R1CS, along with the witness wasm calculator
+let r1cs_path = PathBuf::from("./src/frontend/circom/test_folder/cubic_circuit.r1cs");
+        let wasm_path =
+            PathBuf::from("./src/frontend/circom/test_folder/cubic_circuit_js/cubic_circuit.wasm"); 
+let mut circom_fcircuit = CircomFCircuit::<Fr>::new((r1cs_path, wasm_path, 1, 2)).unwrap(); // state_len:1, external_inputs_len:2
+
+// to speed things up, you can define a custom step function to avoid defaulting to the snarkjs witness calculator 
+circom_fcircuit.set_custom_step_native(Rc::new(|_i, z_i, _external| {
+            let z = z_i[0];
+            Ok(vec![z * z * z + z + Fr::from(5)])
+        }));
+
+// we initialize the required folding schemes parameters, the folding scheme and the final decider that we want to use 
+let (fs_prover_params, kzg_vk, g16_pk, g16_vk) =
+        init_ivc_and_decider_params::<CircomFCircuit<Fr>>(f_circuit.clone());
+
+pub type NOVA = Nova<G1, GVar, G2, GVar2, CircomFCircuit<Fr>, KZG<'static, Bn254>, Pedersen<G2>>;
+pub type DECIDERETH_FCircuit = DeciderEth<
+        G1,
+        GVar,
+        G2,
+        GVar2,
+        CircomFCircuit<Fr>,
+        KZG<'static, Bn254>,
+        Pedersen<G2>,
+        Groth16<Bn254>,
+        NOVA,
+>;
+
+// initialize the folding scheme engine, in our case we use Nova
+let mut nova = NOVA::init(&fs_prover_params, f_circuit.clone(), z_0).unwrap();
+
+// run n steps of the folding iteration
+for (i, external_inputs_at_step) in external_inputs.iter().enumerate() {
+    let start = Instant::now();
+    nova.prove_step(external_inputs_at_step.clone()).unwrap();
+    println!("Nova::prove_step {}: {:?}", i, start.elapsed());
+}
+
+```
+You can find an example for using Nova over a circom circuit [here](https://github.com/privacy-scaling-explorations/sonobe/blob/main/examples/circom_full_flow.rs).