Add documentation for SMS integration (#11894)

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* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

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* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

* Update sms.markdown

* Update sms.markdown

* Update source/_integrations/sms.markdown

Co-Authored-By: Franck Nijhof <frenck@frenck.nl>

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source/_integrations/sms.markdown

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+---
+title: "SMS notifications via GSM modem"
+description: "SMS notification via GSM modem."
+icon: gammu.png
+ha_category:
+  - Notifications
+ha_release: 0.105
+ha_iot_class: Local Polling
+ha_config_flow: false
+ha_codeowners:
+  - '@ocalvo'
+---
+
+The `sms` integration allows having a local execution SMS notification via [Gammu](https://wammu.eu/gammu/). This is ideal when the internet is offline or when the power goes out.
+
+This integration provides the following platforms:
+- Notify
+
+## Configuration
+
+To enable those notifications in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sms:
+  device: /dev/ttyUSB2
+
+notify:
+  - platform: sms
+    name: sms_person1
+    recipient: PHONE_NUMBER
+  - platform: sms
+    name: sms_person2
+    recipient: PHONE_NUMBER
+```
+{% configuration %}
+device:
+  description: The gsm modem device.
+  required: true
+  type: string
+{% endconfiguration %}
+
+To use notifications, please see the [getting started with automation page](/getting-started/automation/).
+
+If the ingegration is used in HassOS then version [3.6](https://github.com/home-assistant/hassos/releases/tag/3.6) or higher is required.
+
+For installations not running on Hass.io or Docker, you must install `gammu-dev` package:
+
+```bash
+sudo apt-get install libgammu-dev
+```
+
+Before running for the first time, check that the modem is recognized by the system by running:
+
+```bash
+ls -l /dev/*USB*
+```
+
+Note: In Hass.io you need to install the SSH add-on.
+
+## Required Hardware
+
+You will need a USB GSM stick modem.
+
+### List of modems known to work
+
+- [Huawei E3372-510](https://www.amazon.com/gp/product/B01N6P3HI2/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1)(
+Need to unlock it using [this guide](http://blog.asiantuntijakaveri.fi/2015/07/convert-huawei-e3372h-153-from.html))
+
+[List of modems that may work](https://www.asus.com/event/networks_3G4G_support/)
+
+### Note about Raspberry PI 4
+
+On Raspberry PI 4, you need a udev rule in the config USB stick, for the [Huawei E3372-510 stick](https://www.amazon.com/gp/product/B01N6P3HI2/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1) for it to be recognized.
+
+Set this content in file `udev\10-gsm-modem.rules` in the configuration USB:
+
+```txt
+ACTION=="add" \
+, ATTRS{idVendor}=="12d1" \
+, ATTRS{idProduct}=="14fe" \
+, RUN+="/sbin/usb_modeswitch -X -v 12d1 -p 14fe"
+```
+
+## More details:
+
+- [Original thread discussion](https://community.home-assistant.io/t/send-sms-with-usb-gsm-modem-when-alarm-triggered/28942/38)