added a note about different NDKs (see #219)

README.md

@@ -63,7 +63,9 @@ Please note:
     APP_CPPFLAGS += -frtti -fexceptions
     ```
 
-- For GCC running on MinGW or Android SDK, the error `'to_string' is not a member of 'std'` (or similarly, for `strtod`) may occur. Note this is not an issue with the code,  but rather with the compiler itself. On Android, see above to build with a newer environment.  For MinGW, please refer to [this site](http://tehsausage.com/mingw-to-string) and [this discussion](https://github.com/nlohmann/json/issues/136) for information on how to fix this bug.
+    The code compiles successfully with [Android NDK](https://developer.android.com/ndk/index.html?hl=ml), Revision 9 - 11 (and possibly later) and [CrystaX's Android NDK](https://www.crystax.net/en/android/ndk) version 10.
+
+- For GCC running on MinGW or Android SDK, the error `'to_string' is not a member of 'std'` (or similarly, for `strtod`) may occur. Note this is not an issue with the code,  but rather with the compiler itself. On Android, see above to build with a newer environment.  For MinGW, please refer to [this site](http://tehsausage.com/mingw-to-string) and [this discussion](https://github.com/nlohmann/json/issues/136) for information on how to fix this bug. For Android NDK using `APP_STL := gnustl_static`, please refer to [this discussion](https://github.com/nlohmann/json/issues/219).
 
 The following compilers are currently used in [continuous integration](https://travis-ci.org/nlohmann/json):