Add cookbook section to docs

docs/cookbook.rst

@@ -0,0 +1,20 @@
+Cookbook
+========
+
+Filtering Records Based on Extra Info
+-------------------------------------
+
+.. code-block:: python
+
+    # This code demonstrates the usage of the `extra` argument for log records to enable advanced filtering of records through handlers
+
+    import logbook
+
+    if __name__ == "__main__":
+
+	only_interesting = logbook.FileHandler('/tmp/interesting.log', filter=lambda r, h: r.extra['interesting'])
+	everything = logbook.FileHandler('/tmp/all.log', bubble=True)
+
+	with only_interesting, everything:
+	    logbook.info('this is interesting', extra={'interesting': True})
+	    logbook.info('this is not interesting')

docs/index.rst

@@ -38,6 +38,7 @@ Documentation
    api/index
    designexplained
    designdefense
+   cookbook
    changelog
 
 Project Information