Mention tags as a valid option for Raven.config

I looked for a way to specify unchanging tags (such as git commit hash of my software), and mistakenly decided it wasn't possible to pass them to Raven.config() because the documentation enumerated various possible options but didn't mention `tags`.

Bugs like #75 convinced me that this was supposed to be a supported feature.  So here's a doc patch to avoid sending other users into confusion.

Author: mgedmin

docs/config/index.rst

@@ -37,6 +37,17 @@ The name of the logger used by Sentry. Default: ``javascript``
 
 .. _config-whitelist-urls:
 
+tags
+----
+
+Additional `tags <https://www.getsentry.com/docs/tags/>`__ to assign to each event.
+
+.. code-block:: javascript
+
+    {
+      tags: {git_commit: 'c0deb10c4'}
+    }
+
 whitelistUrls
 -------------