Adds the catch operator to the README

README.md

@@ -368,6 +368,8 @@ interface Observable {
   Observable take(unsigned long long);
   Observable drop(unsigned long long);
   Observable flatMap(Mapper mapper);
+  Observable catch(Mapper mapper);
+
   Promise<sequence<any>> toArray(optional PromiseOptions options);
   Promise<undefined> forEach(Visitor callback, optional PromiseOptions options);
 
@@ -501,6 +503,10 @@ If the subscriber has already been aborted (i.e., `subscriber.signal.aborted` is
 
 We propose the following operators in addition to the `Observable` interface:
 
+- `catch()`
+  - Like `Promise.catch()`, it takes a callback which gets fired after the source
+    observable errors. It will then map to a new observable, returned by the callback,
+    unless the error is rethrown.
 - `takeUntil(Observable)`
   - Returns an observable that mirrors the one that this method is called on,
     until the input observable emits its first value