docs(operators): add let operator

Author: Wortmann, Jan-Niklas

src/operator-docs/utility/let.ts

@@ -1,6 +1,61 @@
 import { OperatorDoc } from '../operator.model';
 
 export const letOperator: OperatorDoc = {
-  'name': 'let',
-  'operatorType': 'utility'
+  name: 'let',
+  operatorType: 'utility',
+  signature: 'public let(fun: function): Observable,',
+  parameters: [
+    {
+      name: 'fun',
+      type: 'function',
+      attribute: '',
+      description: `Selector function which can use the source sequence
+    as many times as needed, without sharing subscriptions to the source sequence.`
+    }
+  ],
+  shortDescription: {
+    description: `Returns an observable sequence that is the result of invoking the selector on the source sequence,
+    without sharing subscriptions. This operator allows for a fluent style of writing queries that use the same
+    sequence multiple times. There is an alias of letBind for browsers older than IE 9.`
+  },
+  additionalResources: [
+    {
+      url: 'https://www.learnrxjs.io/operators/utility/let.html',
+      description: 'several examples of using the let operator',
+      author: 'btroncone'
+    }
+  ],
+  examples: [
+    {
+      name: 'calls concat as part of a pipeline',
+      code: `
+    const obs = Rx.Observable.range(1, 3);
+
+    const source = obs.let((o) => o.concat(o));
+
+    const subscription = source.subscribe(
+    (x) => {
+        console.log('Next: ' + x);
+    },
+    (err) => {
+        console.log('Error: ' + err);
+    },
+    () => {
+        console.log('Completed');
+    });
+
+    // => Next: 1
+    // => Next: 2
+    // => Next: 3
+    // => Next: 1
+    // => Next: 2
+    // => Next: 3
+    // => Completed
+    `,
+      externalLink: {
+        platform: 'JSBin',
+        url: 'http://jsbin.com/giwanupiqu/embed?html,js,console'
+      }
+    }
+  ]
 };