Update the README to give better instructions for Android.

The MingW link doesn't help for Android, and points people down the
wrong path.  On Android all you need to do is tell the build system
to use a newer compiler and standard library.

The unit tests run successfully with this configuration on Android.

Related to nlohmann#136.

README.md

@@ -53,7 +53,16 @@ I would be happy to learn about other compilers/versions.
 Please note:
 
 - GCC 4.8 does not work because of two bugs ([55817](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55817) and [57824](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57824)) in the C++11 support.
-- 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. 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.
+- Android defaults to using very old compilers and C++ libraries.  To fix this, add the following to your Application.mk.  
+This will switch to the LLVM C++ library, the Clang compiler, and enable C++11 and other features disabled by default.
+
+```
+APP_STL := c++_shared
+NDK_TOOLCHAIN_VERSION := clang3.6
+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.
 
 ## Examples