Add support for emailing results via SMTP

Utilises the new reporter plugin architecture to add support for
emailing success/failure messages based on layout.yaml.

This will assist in testing new gates as currently after a job has
finished if no report is sent back to gerrit then only the workers
logs can be consulted to see if it was successful. This will allow
developers to see exactly what zuul will return if they turn on
gerrit reporting.

Change-Id: I47ac038bbdffb0a0c75f8e63ff6978fd4b4d0a52
diff --git a/doc/source/reporters.rst b/doc/source/reporters.rst
index d64d4f7..18d35a1 100644
--- a/doc/source/reporters.rst
+++ b/doc/source/reporters.rst
@@ -29,4 +29,31 @@
 ~~~~~~~~~~~~~~~~~~~~
 
 The configuration for posting back to gerrit is shared with the gerrit
-trigger in zuul.conf. Please consult the gerrit trigger documentation.
+trigger in zuul.conf as described in :ref:`zuulconf`.
+
+SMTP
+----
+
+A simple email reporter is also available.
+
+SMTP Configuration
+~~~~~~~~~~~~~~~~~~
+
+zuul.conf contains the smtp server and default to/from as describe
+in :ref:`zuulconf`.
+
+Each pipeline can overwrite the to or from address by providing
+alternatives as arguments to the reporter. For example, ::
+
+  pipelines:
+    - name: post-merge
+      manager: IndependentPipelineManager
+      trigger:
+        - event: change-merged
+      success:
+        smtp:
+          to: you@example.com
+      failure:
+        smtp:
+          to: you@example.com
+          from: alternative@example.com