Update Android ndk-build usage

PluginSource/projects/Android/README.md

@@ -0,0 +1,40 @@
+
+# Android Build Instructions
+
+### Setup Environment Variables
+```sh
+# install so file to unity project
+export NDK_LIBS_OUT="~/workspaecs/NativeRenderingPlugin/UnityProject/Assets/Plugins/Android/libs"
+```
+
+
+### Build
+```sh
+# windows
+$NDK21/ndk-build.cmd
+
+
+# linux/mac
+$NDK21/ndk-build
+```
+
+`NDK21` is the path to your NDK installation. For example, `~/Library/Android/sdk/ndk/21.0.6113669` on macOS.
+
+
+Finally output:
+```sh
+.
+└── libs
+    ├── arm64-v8a
+    │   ├── libc++_shared.so     
+    │   └── libRenderingPlugin.so
+    ├── armeabi-v7a
+    │   ├── libc++_shared.so     
+    │   └── libRenderingPlugin.so
+    ├── x86
+    │   ├── libc++_shared.so     
+    │   └── libRenderingPlugin.so
+    └── x86_64
+        ├── libc++_shared.so     
+        └── libRenderingPlugin.so
+```

PluginSource/projects/Android/jni/Application.mk

@@ -1,5 +1,5 @@
 APP_ABI := armeabi-v7a arm64-v8a x86 x86_64
-APP_PLATFORM := android-9
-APP_STL := gnustl_static
+APP_PLATFORM := android-16
+APP_STL := c++_shared
 APP_CPPFLAGS += -std=c++11
 NDK_TOOLCHAIN_VERSION := clang