Update README

README.md

@@ -18,8 +18,12 @@ We recommend you checkout the [docs](https://docs.rs/factrs/latest/factrs/) (WIP
 ```rust
 use factrs::prelude::*;
 
+// Assign symbols to variable types
+assign_symbols!(X: SO2);
+
 // Make all the values
 let mut values = Values::new();
+
 let x = SO2::from_theta(1.0);
 let y = SO2::from_theta(2.0);
 values.insert(X(0), SO2::identity());
@@ -28,13 +32,16 @@ values.insert(X(1), SO2::identity());
 // Make the factors & insert into graph
 let mut graph = Graph::new();
 let res = PriorResidual::new(x.clone());
-let factor = Factor::new_base(&[X(0)], res);
+let factor = FactorBuilder::new1(res, X(0)).build();
 graph.add_factor(factor);
 
 let res = BetweenResidual::new(y.minus(&x));
 let noise = GaussianNoise::from_scalar_sigma(0.1);
 let robust = Huber::default();
-let factor = Factor::new_full(&[X(0), X(1)], res, noise, robust);
+let factor = FactorBuilder::new2(res, X(0), X(1))
+    .noise(noise)
+    .robust(robust)
+    .build();
 graph.add_factor(factor);
 
 // Optimize!