Add blog post about template and streaming (#718)

Co-authored-by: Franck Nijhof <git@frenck.dev>

blog/2020-11-09-system-health-and-templates.md

@@ -0,0 +1,59 @@
+---
+author: Paulus Schoutsen
+authorURL: https://twitter.com/balloob
+authorImageURL: /img/profile/paulus.jpg
+authorTwitter: balloob
+title: System Health and Templates
+---
+
+In Home Assistant 0.118, there will be two changes that could impact your custom integration.
+
+## Removed deprecated `helpers.template.extract_entities`
+
+The previously deprecated `extract_entities` method from the Template helper has been removed ([PR 42601](https://github.com/home-assistant/core/pull/42601)). Instead of extracting entities and then manually listen for state changes, use the new `async_track_template_result` from the Event helper. It will dynamically make sure that every touched entity is tracked correctly.
+
+```python
+from homeassistant.helpers.event import async_track_template_result, TrackTemplate
+
+template = "{{ light.kitchen.state == 'on' }}"
+
+async_track_template_result(
+    hass,
+    [TrackTemplate(template, None)],
+    lambda event, updates: print(event, updates),
+)
+```
+
+## Improved System Health
+
+Starting with Home Assistant 0.118, we're deprecating the old way of providing system health information for your integration. Instead, create a `system_health.py` file in your integration ([PR 42785](https://github.com/home-assistant/core/pull/42785)).
+
+Starting this release, you can also include health checks that take longer to resolve ([PR 42831](https://github.com/home-assistant/core/pull/42831)), like checking if the service is online. The results will be passed to the frontend when they are ready.
+
+```python
+"""Provide info to system health."""
+from homeassistant.components import system_health
+from homeassistant.core import HomeAssistant, callback
+
+from .const import DOMAIN
+
+
+@callback
+def async_register(
+    hass: HomeAssistant, register: system_health.RegisterSystemHealth
+) -> None:
+    """Register system health callbacks."""
+    register.async_register_info(system_health_info)
+
+
+async def system_health_info(hass):
+    """Get info for the info page."""
+    client = hass.data[DOMAIN]
+
+    return {
+      "server_version": client.server_version,
+      "can_reach_server": system_health.async_check_can_reach_url(
+          hass, client.server_url
+      )
+    }
+```