Minimum ios version

Hi @EddyVerbruggen, 
The work you've made on this project, is simply amazing.  Thanks a lot.
I am a new a newbee in nativescript, but I have struggle with a compilation issue.
If you think this additional line can be useful. otherwise, no problem and thanks again for your great job.

Author: nickykln

docs/ML_KIT.md

@@ -32,6 +32,11 @@ and add this somewhere in the file (if it's not already there):
   <key>NSCameraUsageDescription</key>
   <string>Your reason here</string> <!-- better change this 😎 -->
 ```
+In order to compile, the default version on ios must be >= 9.0. 
+Edit the file build.xconfig and check that you have the following line (Without this line the default version will be 8.0 and the compilation will failed (`targeted OS version does not support use of thread local variables ...`)
+```
+IPHONEOS_DEPLOYMENT_TARGET = 9.0;
+```
 
 ## ML Kit Features
 There are two ways of using ML Kit: