Create advanced-usage.md (#11)

* Create advanced-usage.md

* Update README.md

* Update advanced-usage.md

* Update README.md

README.md

@@ -5,6 +5,8 @@ For more information on how to build HTML In-App Messages view our [documentatio
 
 ## Templates
 
+For more information of how html in-apps work see our [advanced usage doc](./advanced-usage.md)
+
 ###### [Leads (SMS + Email)](./sms_email_form/README.md)
 <img alt="Leads" src="./sms_email_form/readme_assets/sms_email_form.gif" width="200px">
 
@@ -16,8 +18,3 @@ For more information on how to build HTML In-App Messages view our [documentatio
 
 ###### [Promo Wheel](./promo_wheel/README.md)
 <img alt="Promo Wheel" src="./promo_wheel/readme_assets/promo_wheel_iam.gif" width="200px">
-
-### Safe Area (notch) Detection
-Certain mobile devices may have safe areas where operating system icons or a camera notch might exist. Our Onesignal SDK detects these areas and exposes them to the iam's HTML document via `safe-area-inset-*` CSS variables :point_down:
-
-https://github.com/OneSignalDevelopers/iam-html-templates/blob/00e30f0240e22f2e72ae807ac9274c3d0ab97bc8/sms_email_form/index.html#L21-L31

advanced-usage.md

@@ -0,0 +1,4 @@
+### Safe Area (notch) Detection
+Certain mobile devices may have safe areas where operating system icons or a camera notch might exist. Our Onesignal SDK detects these areas and exposes them to the iam's HTML document via `safe-area-inset-*` CSS variables :point_down:
+
+https://github.com/OneSignalDevelopers/iam-html-templates/blob/00e30f0240e22f2e72ae807ac9274c3d0ab97bc8/sms_email_form/index.html#L21-L31