Updating readme to explain how to exclude files from sitemap

Updating readme to explain how to exclude files from sitemap.

sitemap/Readme.rst

@@ -31,6 +31,8 @@ The ``SITEMAP`` variable must be a Python dictionary and can contain three keys:
 
   Valid frequency values are ``always``, ``hourly``, ``daily``, ``weekly``, ``monthly``,
   ``yearly`` and ``never``.
+  
+You can exclude URLs from being included in the sitemap by adding them to the sitemapExclude array in format 'sitemapExclude = ['login.html', 'signup.html']`.  
 
 If a key is missing or a value is incorrect, it will be replaced with the
 default value.