Add explanation of the new template rate_limit directive.

source/_docs/configuration/templating.markdown

@@ -58,7 +58,7 @@ Extensions allow templates to access all of the Home Assistant specific states a
 - `is_state('device_tracker.paulus', 'home')` will test if the given entity is the specified state.
 - `state_attr('device_tracker.paulus', 'battery')` will return the value of the attribute or None if it doesn't exist.
 - `is_state_attr('device_tracker.paulus', 'battery', 40)` will test if the given entity attribute is the specified state (in this case, a numeric value). Note that the attribute can be `None` and you want to check if it is `None`, you need to use `state_attr('sensor.my_sensor', 'attr') == None`. 
-
+- `rate_limit` - limits re-renders of the template. Returns an empty string and accepts the same arguments as the Python `datetime.timedelta` function -- days, seconds, microseconds, milliseconds, minutes, hours, weeks.
 <div class='note warning'>
 
   Avoid using `states.sensor.temperature.state`, instead use `states('sensor.temperature')`. It is strongly advised to use the `states()`, `is_state()`, `state_attr()` and `is_state_attr()` as much as possible, to avoid errors and error message when the entity isn't ready yet (e.g., during Home Assistant startup).

source/_integrations/binary_sensor.template.markdown

@@ -294,6 +294,31 @@ binary_sensor:
 
 {% endraw %}
 
+### Rate limiting updates
+
+When there are entities present in the template, the template will be re-rendered when one of the entities changes states.
+
+When `states` is used in a template by itself to iterate all states on the system, the template is re-rendered each
+time any state changed event happens if any part of the state is accessed. When merely counting states, the template
+is only re-rendered when a state is added or removed from the system. On busy systems with many entities or hundreds of
+thousands state changed events per day, templates may re-render more than desirable.
+
+A `rate_limit` directive can be used to limit how often the template re-renders.
+
+`rate_limit` returns an empty string and accepts the same arguments as the Python `datetime.timedelta` function -- days, seconds, microseconds, milliseconds, minutes, hours, weeks.
+
+In the below example, re-renders are limited to once per minute:
+
+{% raw %}
+```yaml
+binary_sensor:
+  - platform: template
+    sensors:
+      has_unavailable_states:
+        value_template: '{{ rate_limit(minutes=1) }}{% states | selectattr('state', 'in', ['unavailable', 'unknown', 'none']) | list | count }}'
+```
+{% endraw %}
+
 ### Working without entities
 
 The `template` sensors are not limited to use attributes from other entities but can also work with [Home Assistant's template extensions](/docs/configuration/templating/#home-assistant-template-extensions). If the template includes some non-deterministic property such as time in its calculation, the result will not continually update, but will only update when some entity referenced by the template updates.