Add steamist integration (#20995)

source/_integrations/steamist.markdown

@@ -0,0 +1,23 @@
+---
+title: Steamist
+description: Documentation about Steamist steam showers.
+ha_category:
+  - Switch
+ha_iot_class: Local Polling
+ha_release: 2022.2
+ha_config_flow: true
+ha_codeowners:
+  - '@bdraco'
+ha_domain: steamist
+ha_platforms:
+  - switch
+---
+
+The Steamist will allow you to monitor the state of your [Steamist](https://steamist.com/digital-controls/) steam shower and turn on and off the steam generator.
+
+## Supported Devices
+
+- 450 Digital Control
+- 550 Digital Control
+
+{% include integrations/config_flow.md %}