Add guide to build wams library

Signed-off-by: Le Yao <le.yao@intel.com>

modsecurity2wasm.md

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+# Build ModSecurity WASM Library
+
+This article will guide you to build your own ModSecurity WASM library using [Emscripten](https://emscripten.org/) toolchain.
+
+## Pre-requirements
+
+### Install Emscripten
+
+You can refer to the following steps to install the latest `Emscripten`.
+
+```shell
+# Get the emsdk repo
+git clone https://github.com/emscripten-core/emsdk.git
+
+# Enter that directory
+cd emsdk
+
+# Fetch the latest version of the emsdk (not needed the first time you clone)
+git pull
+
+# Download and install the latest SDK tools.
+./emsdk install latest
+
+# Make the "latest" SDK "active" for the current user. (writes .emscripten file)
+./emsdk activate latest
+
+# Activate PATH and other environment variables in the current terminal
+source ./emsdk_env.sh
+```
+
+
+
+### `wasi-sdk` setup
+
+- Download 
+
+  ```shell
+  wget https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-12/wasi-sdk-12.0-linux.tar.gz
+  ```
+
+- Export it to `/opt/wasi-sdk`
+
+- Configure
+
+  ```shell
+  export WASI_SDK_PATH="/opt/wasi-sdk"
+  ```
+
+
+
+### Build PCRE WASM library
+
+```shell
+# Get the pcre library source code
+git clone https://github.com/maxfierke/libpcre.git -b mf-wasm32-wasi-cross-compile
+
+cd libpcre
+# This should compile successfully and place the compiled .a static library in targets/wasm32-wasi
+Run ./build_for_crystal.sh. 
+
+# Copy the wams library to target directory
+cp targets/wasm32-wasi/*.a /usr/local/pcre
+```
+
+
+
+## Configure and build ModSecurity
+
+```shell
+# This is version for WASM ModSecurity
+git clone https://github.com/leyao-daily/ModSecurity.git
+
+cd ModSecurity
+# Build the configuration script
+./build.sh
+
+# Download the submodule
+git submodule init
+git submodule update
+
+# Configure ModSecurity with core functions
+emconfigure ./configure --without-yajl --without-geoip --without-libxml --without-curl --disable-shared --disable-examples --disable-libtool-lock --disable-debug-logs --with-pcre=./pcre-config
+
+# Build the library
+emmake make -j <num_cpus>
+
+# Install the library
+emmake make install
+
+```
+
+
+
+## Build your own wasm application
+
+```sehll
+emcc test.cc -L/usr/local/modsecurity/lib/ -lmodsecurity -L/usr/local/pcre/ -lpcre -o test.wasm -I/usr/local/modsecurity/include/
+```
+