add iOS build instructions

docs/ios.mdx

@@ -0,0 +1,12 @@
+## iOS
+
+### Min iOS version
+
+Thermion requires a minimum iOS version of 13.0
+
+In your application's `ios/Podfile`, make sure the following line is present (and not commented out):
+
+```ruby
+platform :ios, '13.0'
+```
+