Add blog post about core.async_get_hass() (#1443)

blog/2022-08-24-globally_accessible_hass.md

@@ -0,0 +1,20 @@
+---
+author: Erik Montnemery
+authorURL: https://github.com/emontnemery
+title: "Globally available HomeAssistant object (hass)"
+---
+
+It's now possible to get a reference to the `HomeAssistant` instance by calling `core.async_get_hass()`.
+
+Although this means it's no longer strictly necessary to pass `hass` around, the recommendation is still to only use `core.async_get_hass` where it's very cumbersome or downright impossible to pass `hass` to the code which needs it.
+An example where this can be useful is voluptuous validators, which previously couldn't access `hass` because voluptuous has no way of passing user data to validators.
+
+```python
+@callback
+def async_get_hass() -> HomeAssistant:
+    """Return the HomeAssistant instance.
+    Raises LookupError if no HomeAssistant instance is available.
+    This should be used where it's very cumbersome or downright impossible to pass
+    hass to the code which needs it.
+    """
+```