Add JavaScript example

README.md

@@ -7,7 +7,9 @@
 npm install sums-up
 ```
 
-### Example:
+### Example
+
+In TypeScript:
 
 ```typescript
 import SumType from 'sums-up';
@@ -30,6 +32,29 @@ const result = x.caseOf({
 });
 ```
 
+Or in JavaScript
+
+```javascript
+import SumType from 'sums-up';
+
+class Maybe extends SumType {}
+
+function Just(value) {
+  return new Maybe("Just", value);
+}
+
+function Nothing() {
+  return new Maybe("Nothing");
+}
+
+const x = Just("foo");
+
+const result = x.caseOf({
+  Nothing: () => "nope",
+  Just: (a) => a + "bar",
+});
+```
+
 ### Wildcard Matching
 
 If the kind of a sum type instance isn't present in the pattern given to `caseOf`, a default key called `_` will be used instead.