Add link to docs

[skip ci]

Author: ruslo

os/android.cmake

@@ -32,7 +32,10 @@ endif()
 set(CMAKE_SYSTEM_NAME "Android")
 
 if(CMAKE_VERSION VERSION_LESS 3.7.1)
-  polly_fatal_error("Minimum CMake version for Android is 3.7.1")
+  polly_fatal_error(
+      "Minimum CMake version for Android is 3.7.1:"
+      "* http://polly.readthedocs.io/en/latest/toolchains/android.html#android-ndk-x-api-y"
+  )
 endif()
 
 macro(find_host_program)