Add property watches

Property watches cache DBus property values given an externally
supplied index of property names and paths, in an externally
supplied storage location.

Change-Id: I155081da88c3ab0e4f6a13b012fc9719203b1888
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/src/example/example.yaml b/src/example/example.yaml
index 3d60396..4e216cd 100644
--- a/src/example/example.yaml
+++ b/src/example/example.yaml
@@ -44,3 +44,12 @@
     - interface: xyz.openbmc_project.Sensor.Value
       meta: property
       property: ValueB
+
+- name: example property watch
+  description: >
+    'A property watch instructs PDM to maintain a cache of the state
+    of the specified properties on the specified DBus objects.'
+  class: watch
+  watch: property
+  paths: example path group
+  properties: example property group