Create SMS documentation

source/_integrations/sms.markdown

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+# Local SMS notifications integration via GSM Modem
+This integration allows to have a local execution SMS notification. This is ideal when the internet is offline or when the power goes out.
+
+# 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/)
+
+# Configuration
+It requires hassos version [3.6](https://github.com/home-assistant/hassos/releases/tag/3.6) or higher.
+Check that the modem is recognized by running:
+```
+ls -l /dev/*USB*
+```
+
+## Home Assistant config
+In home assistant you need to configure your yaml notification as:
+```
+notify:
+  - platform: command_line
+    name: sms_person1
+    command: !secret notify_sms_person1
+  - platform: command_line
+    name: sms_person2
+    command: !secret notify_sms_person2
+```
+And the screts yaml:
+```
+notify_sms_person1: "/config/custom_components/sms/send_sms.sh +1NNNNNNNNNN /dev/ttyUSB2"
+notify_sms_person2: "/config/custom_components/sms/send_sms.sh +1NNNNNNNNNN /dev/ttyUSB2"
+```
+Replace the NNN for the actual phone numbers
+
+## Note about Raspberry
+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.
+This config is:
+```
+udev\10-gsm-modem.rules
+```
+```
+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)