v3.2.0: Added docs on let helper

Author: Kenneth Larsen

guides/v3.2.0/templates/built-in-helpers.md

@@ -36,3 +36,32 @@ format of a concatenated string.
 
 This will display the result of `this.get('foo.item1')` when index is 1,
 and `this.get('foo.item2')` when index is 2, etc.
+
+### Built-in block helpers
+Now let's say your template is starting to get a bit cluttered and you now want to clean up the logic in your templates. This can be achieved with the `let` block helper. The [`{{let}}`](#) helper lets you create new bindings in your template.
+
+Say your template now looks like this:
+
+```handlebars
+Welcome back {{concat (capitalize person.firstName) ' ' (capitalize person.lastName)}}
+
+Account Details:
+First Name: {{capitalize person.firstName}}
+Last Name: {{capitalize person.lastName}}
+```
+
+As mentioned in the previous section we use the `concat` helper to render both `person.firstName` and `person.lastName` in one go. But we also want to make sure that the names are capitalized. It gets a bit repetitive to keep writing `capitalize` and honestly, we might just forget it at some point. Thankfully, we can use the `{{let}}` helper to fix this:
+
+```handlebars
+{{#let (capitalize person.firstName) (capitalize person.lastName)
+  as |firstName lastName|
+}}
+  Welcome back {{concat firstName ' ' lastName}}
+
+  Account Details:
+  First Name: {{firstName}}
+  Last Name: {{lastName}}
+{{/let}}
+```
+
+Now, as long as your template is wrapped in the `let` helper you can access the capitalized first name and last name as `firstName` and `lastName` instead of `(capitalize person.firstName)`.