Add the first draft of SW high level design (#921)

* Add the first draft of SW high level design

This commit adds the first draft of SW high level design.

Signed-off-by: Sangmin Seo <sangmin7.seo@samsung.com>

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+# Software High Level Design
+
+## Design
+
+### SW System Overall Architecture
+
+![nnfw_architecture](../fig/nnfw_architecture.png)
+
+The figure above illustrates the overall software stack including _nnfw_, which consists of ML
+Framework and NN Runtime, and NN Compute.  Note that NN Compute is provided by the underlying
+platform, and NN Runtime utilizes NN Compute for operation acceleration.  The next bullets describe
+the role of each module and the background of design choice in the architecture.
+
+1. ML Framework
+   - Provide TensorFlow (TF) Lite on Tizen and SMP
+      - We chose TF Lite as a standard ML framework in _nnfw_ for this year, since TF Lite is
+        lightweight compared to other ML frameworks and its community is rapidly growing. We expect
+        supporting TF Lite on Samsung's OS platforms would be beneficial to Samsung's diverse
+        business areas and AI solutions.
+   - Provide TF Lite C# API for Tizen .NET
+      - Considering the existing TF Lite supports only C++ and Java API, C# API for TF Lite would
+        be a great complement to TF Lite and natural extension for Tizen.
+1. NN Runtime
+   - Provide a common runtime interface, which is Android NN API
+      - Android NN API (NN API for short) was selected for seamless integration with TF Lite. As
+        long as our NN runtime provides NN API as an interface, TF Lite can link to our NN runtime
+        without any modification.
+      - Although we borrowed NN API as the runtime's interface, we plan to design and implement the
+        runtime itself by ourselves. For the implementation, we will utilize ARM Compute Library
+        (ACL) for NN operation acceleration on ARM CPU and GPU.
+1. NN Compute
+   - Provide computation acceleration library, such as ACL, or device driver for NPU
+   - This layer will be provided by OS platform, and we will use the library or device driver as it
+     is. We may request a specific version to the Platform team, but we don't expect we will be
+     modifying the library.
+
+
+### SW Structure Design
+
+1. ML Framework
+   - Provide TensorFlow (TF) Lite on Tizen and SMP
+   - Provide TF Lite C# API for Tizen .NET
+1. NN Runtime
+   - Provide an implementation of Android NN API
+   - Provide hardware acceleration of NN operations on the target platform
+
+
+### SW Behavior Design
+
+![nnfw_behavior](../fig/nnfw_behavior.png)
+
+The figure above depicts the execution flow from the user input to the inference result output.
+
+1. Input is a TF Lite model and is fed into TF Lite.  Application on Tizen or SMP may use C# or C++
+   API to load and run the input model.
+1. TF Lite determines whether the NN runtime is provided and supported on the platform or not.  If
+   supported, it constructs an internal model from the TF Lite model for the NN runtime and passes
+   the model to the runtime.  Otherwise, it invokes the internal interpreter, which runs on CPU, in
+   order to perform inference with the given model.
+1. If the NN runtime receives the model from TF Lite, it consults with the Execution Planner about
+   how to decompose operations in the model and how to make an execution order of the operations.
+   The Execution Planner also decides which backend between CPU fallback and ACL kernel backend
+   could be better performing depending on operations.
+1. When the NN runtime finishes the inference on CPU or GPU (or on both), it returns the output to
+   TF Lite, which again delivers the output to the application.
+
+
+### SW Interface Design
+
+1. ML Framework
+   - Java and C++ API of TF Lite will be provided as it is.
+   - C# API will be defined as the project makes progress.
+1. NN Runtime
+   - Public API for NN Runtime is the same as Android NN API that is provided in [Android
+   oreo-m2-release](https://android.googlesource.com/platform/frameworks/ml/+/oreo-m2-release).
+   - The API is defined in
+   [NeuralNetworks.h](https://github.sec.samsung.net/STAR/nnfw/blob/master/src/runtime/ref/nn/runtime/include/NeuralNetworks.h).
+