add core example

packages/core/README.md

@@ -7,3 +7,30 @@ This package contains the bare essence of strudel.
 ```sh
 npm i @strudel.cycles/core --save
 ```
+
+## Example
+
+```js
+import { sequence, State, TimeSpan } from '@strudel.cycles/core';
+
+const pattern = sequence('a', ['b', 'c']);
+
+const events = pattern.query(new State(new TimeSpan(0, 2)));
+
+const spans = events.map(
+  (event) => `${event.value}: ${event.whole.begin.toFraction()} - ${event.whole.end.toFraction()} `,
+);
+```
+
+spans:
+
+```log
+a: 0 - 1/2
+b: 1/2 - 3/4
+c: 3/4 - 1
+a: 1 - 3/2
+b: 3/2 - 7/4
+c: 7/4 - 2
+```
+
+[play with @strudel.cycles/core on codesandbox](https://codesandbox.io/s/strudel-core-test-qmz6qr?file=/src/index.js).