Make web app manifest file comments more clear

Update inline comments regarding the web application manifest file to
point out that it is recommended (not mandatory) that this file should
be serve with the `application/manifest+json` media type.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

From http://w3c.github.io/manifest/:

  " If the protocol over which the manifest is transferred supports
    the [MIME-TYPES] specification (e.g. HTTP), it is RECOMMENDED that
    the manifest be labeled with the media type for a manifest. "

Thanks @marcoscaceres for pointing that out!

Ref: #29 (comment)

src/.htaccess

@@ -140,11 +140,12 @@ Options -MultiViews
 
   # Manifest files
 
-    # Web application manifest file
-    # http://w3c.github.io/manifest/
+    # If you are providing a web application manifest file (see the
+    # specification: http://w3c.github.io/manifest/), it is recommended
+    # that you serve it with the `application/manifest+json` media type.
     #
-    # Because the web application manifest file doesn't have its own unique
-    # file extension, you can set its media type either by matching:
+    # Because the web application manifest file doesn't have its own
+    # unique file extension, you can set its media type either by matching:
     #
     # 1) the exact location of the file (this can be done using a directive
     #    such as `<Location>`, but it will NOT work in the `.htaccess` file,