Update 17.2 to allow comments in the first line of a block

In #483, justjake said that we should update the style guide to allow
single line comments in the first line of a block.

README.md

@@ -1248,7 +1248,7 @@ Other Style Guides
     }
     ```
 
-  - [17.2](#17.2) <a name='17.2'></a> Use `//` for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment.
+  - [17.2](#17.2) <a name='17.2'></a> Use `//` for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it's on the first line of a block.
 
     ```javascript
     // bad
@@ -1276,6 +1276,14 @@ Other Style Guides
 
       return type;
     }
+
+    // also good
+    function getType() {
+      // set the default type to 'no type'
+      const type = this._type || 'no type';
+
+      return type;
+    }
     ```
 
   - [17.3](#17.3) <a name='17.3'></a> Prefixing your comments with `FIXME` or `TODO` helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are `FIXME -- need to figure this out` or `TODO -- need to implement`.