example: add calc example from readme

Cargo.toml

@@ -30,3 +30,7 @@ vec = []
 
 [badges]
 maintenance = { status = "actively-developed" }
+
+[[example]]
+name = "calc"
+required-features = ["vec", "number"]

examples/calc.rs

@@ -0,0 +1,15 @@
+use estring::{EString, SepVec};
+
+type PlusVec<T> = SepVec<T, '+'>;
+type MulVec<T> = SepVec<T, '*'>;
+
+fn main() -> Result<(), estring::ParseError> {
+    let res = EString::from("10+5*2+3")
+        .parse::<PlusVec<MulVec<f32>>>()?
+        .iter()
+        .map(|m| m.iter().product::<f32>())
+        .sum::<f32>();
+
+    assert_eq!(res, 23.0);
+    Ok(())
+}