Add thendo.md

Author: 38elements

doc/api/core/operators/thendo.md

@@ -0,0 +1,61 @@
+### `Rx.Observable.prototype.thenDo(selector)`
+[Ⓢ](https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/thendo.js "View in source")
+
+Matches when the observable sequence has an available value and projects the value.
+
+#### Arguments
+1. `selector` *(`Function`)*: Selector that will be invoked for values in the source sequence.
+
+#### Returns
+*(`Plan`)*: Plan that produces the projected values, to be fed (with other plans) to the when operator.
+
+#### Example
+```js
+var selector = function (x, y) { return x + ", " + y; };
+
+var source = Rx.Observable.when(
+    Rx.Observable.interval(250).and(Rx.Observable.of("A", "B", "C")).thenDo(selector),
+    Rx.Observable.interval(300).and(Rx.Observable.of("a", "b")).thenDo(selector)
+);
+
+var subscription = source.subscribe(
+    function (x) {
+        console.log('Next: ' + x);
+    },
+    function (err) {
+        console.log('Error: ' + err);
+    },
+    function () {
+        console.log('Completed');
+    });
+
+// => Next: 0, A 
+// => Next: 0, a
+// => Next: 1, B
+// => Next: 1, b
+// => Next: 2, C
+// => Completed
+```
+
+### Location
+
+File:
+- [/src/core/linq/observable/thendo.js](https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/thendo.js)
+
+Dist:
+- [`rx.all.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.all.js)
+- [`rx.all.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.all.compat.js)
+- [`rx.joinpatterns.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.joinpatterns.js)
+
+Prerequisites:
+- [`rx.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js) | [`rx.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.compat.js) | [`rx.lite.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.lite.js) | [`rx.lite.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.lite.compat.js)
+
+NPM Packages:
+- [`rx`](https://www.npmjs.org/package/rx)
+
+NuGet Packages:
+- [`RxJS-All`](http://www.nuget.org/packages/RxJS-All)
+- [`RxJS-JoinPatterns`](http://www.nuget.org/packages/RxJS-JoinPatterns)
+
+Unit Tests:
+- [/tests/observable/when.js](https://github.com/Reactive-Extensions/RxJS/blob/master/tests/observable/when.js)