docs(operators): add documentation for takeWhile

Close #103

Author: ashwin-sureshkumar

src/operator-docs/filtering/takeWhile.ts

@@ -1,6 +1,63 @@
 import { OperatorDoc } from '../operator.model';
 
 export const takeWhile: OperatorDoc = {
-  'name': 'takeWhile',
-  'operatorType': 'filtering'
+  name: 'takeWhile',
+  operatorType: 'filtering',
+  signature:
+    'public takeWhile(predicate: function(value: T, index: number): boolean): Observable<T>',
+  parameters: [
+    {
+      name: 'predicate',
+      type: 'function(value: T, index: number): boolean',
+      attribute: '',
+      description: `
+        A function that evaluates a value emitted by the source Observable and returns a boolean.
+        Also takes the (zero-based) index as the second argument.
+      `
+    }
+  ],
+  marbleUrl: 'http://reactivex.io/rxjs/img/takeWhile.png',
+  shortDescription: {
+    description: `
+    Emits values emitted by the source Observable so long as each value satisfies the given predicate,
+     and then completes as soon as this predicate is not satisfied.
+    `
+  },
+  walkthrough: {
+    description: `
+    <p>
+      <span class="markdown-code">takeWhile</span> subscribes and begins mirroring the source Observable.
+    </p>
+    <p>
+      Each value emitted on the source is given to the predicate function which returns a boolean,
+      representing a condition to be satisfied by the source values.
+    </p>
+    <p>
+      The output Observable emits the source values until such time as the predicate returns false,
+      at which point <span class="takeWhile"> stops mirroring the source Observable and completes the output Observable.
+    </p>
+    `
+  },
+  examples: [
+    {
+      name: `
+      Emit click events only while the clientX property is greater than 100
+      `,
+      code: `
+      import { fromEvent } from 'rxjs/observable/fromEvent';
+      import { takeWhile } from 'rxjs/operators';
+
+      const clicks = fromEvent(document, 'click');
+      const result = clicks.pipe(
+                        takeWhile(ev => ev.clientX > 100)
+                     );
+      result.subscribe(x => console.log(x));
+      `,
+      externalLink: {
+        platform: 'JSBin',
+        url: 'http://jsbin.com/lasosinudi/embed?js,console,output'
+      }
+    }
+  ],
+  relatedOperators: ['take', 'takeLast', 'takeUntil', 'skip']
 };