Integrate WASI-NN into WAMR (#1521)

Initial integration of WASI-NN based on #1225:
- Implement the library core/iwasm/libraries/wasi-nn
- Support TensorFlow, CPU, F32 at the first stage
- Add cmake variable `-DWAMR_BUILD_WASI_NN`
- Add test case based on Docker image and update document

Refer to #1573

core/iwasm/libraries/wasi-nn/README.md

@@ -0,0 +1,43 @@
+# WASI-NN
+
+## How to use
+
+Enable WASI-NN in the WAMR by spefiying it in the cmake building configuration as follows,
+
+```
+set (WAMR_BUILD_WASI_NN  1)
+```
+
+The definition of the functions provided by WASI-NN is in the header file `core/iwasm/libraries/wasi-nn/wasi_nn.h`.
+
+By only including this file in your WASM application you will bind WASI-NN into your module.
+
+## Tests
+
+To run the tests we assume that the current directory is the root of the repository.
+
+
+1. Build the docker image,
+
+```
+docker build -t wasi-nn -f core/iwasm/libraries/wasi-nn/test/Dockerfile .
+```
+
+2. Run the container
+
+```
+docker run wasi-nn
+```
+
+If all the tests have run properly you will the the following message in the terminal,
+
+```
+Tests: passed!
+```
+
+## What is missing
+
+* Only 1 model at a time is supported.
+    * `graph` and `graph-execution-context` are ignored.
+* Only `tensorflow` (lite) is supported.
+* Only `cpu` is supported.