doc(doc/build.md): add description of building on linux.

Author: SanftMonster

doc/Build.md

@@ -3,9 +3,32 @@
 1. Install ```mingw-w64``` and set the enviroment variable.
 2. clone the project.
 3. use the following command
+```
+$ mkdir build
+$ cd build
+$ cmake .. -G "MinGW Makefiles"
+$ mingw32-make
     ```
-    $ mkdir build
-    $ cd build
-    $ cmake .. -G "MinGW Makefiles"
-    $ mingw32-make
-    ```
+    
+## Cmake build on Linux
+
+1. clone the project.
+2. use the following command
+```
+$ mkdir build
+$ cd build
+$ cmake ..
+$ make
+```
+
+## Cmake cross-build on Linux targeting windows
+
+1. Install ```mingw-w64``` and set the enviroment variable.
+2. clone the project.
+3. use the following command
+```
+$ mkdir build
+$ cd build
+$ cmake .. -DNN_CROSS_COMPILE=ON
+$ make
+```