Add tutorial_dnn_build

modules/dnn/tutorials/tutorial_dnn_build.markdown

@@ -0,0 +1,60 @@
+Build dnn module {#tutorial_dnn_build}
+================
+
+Introduction
+------------
+opencv_dnn module is placed in the secondary [opencv_contrib](https://github.com/Itseez/opencv_contrib) repositroy,
+which isn't distributed in binary form, therefore you need to build it manually.
+
+To do this you need to have installed: [CMake](http://www.cmake.org/download), git, and build system (*gcc* with *make* for Linux or *MS Visual Studio* for Windows)
+
+Steps
+-----
+-# Make any directory, for example **opencv_root**
+
+-# Clone [opencv](https://github.com/Itseez/opencv) and [opencv_contrib](https://github.com/Itseez/opencv_contrib) repos to the **opencv_root**.
+   You can do it in terminal like here:
+@code
+cd opencv_root
+git clone https://github.com/Itseez/opencv
+git clone https://github.com/Itseez/opencv_contrib
+@endcode
+
+-# Run [CMake-gui](https://www.johnlamp.net/cmake-tutorial-3-gui-tool.html) and set source and build directories:
+
+    - Set source code directory to **opencv_root**/opencv;
+
+    - Set binaries directory, for exmaple, to **opencv_root**/build_opencv.
+      This directory will contain built libraries.
+
+-# Configure opencv:
+       - press *Configure*;
+       - choose the preffered project generator (Makefiles for Linux, MS Visual Studio for Windows);
+       - also you can set many opencv build options, for more details see @ref tutorial_linux_install.
+
+-# In the appeared list of variables find variable `OPENCV_EXTRA_MODULES_PATH` and set it to the **opencv_root**/opencv_contrib.
+
+-# *Configure* the project again, and set build options of dnn module:
+
+    - `BUILD_opencv_dnn` variable must exist and be checked.
+
+    - dnn module covers waste part of [Caffe](http://caffe.berkeleyvision.org) framework functionality.
+      However, to load Caffe networks libprotobuf is required.
+      You you can uncheck `BUILD_LIBPROTOBUF_FROM_SOURCES` flag to try use protobuf installed in your system.
+      Elsewise libpotobuf will be built from opencv sources.
+
+    - You can additionaly check `opencv_dnn_BUILD_TORCH_IMPORTER` variable to build [Torch7](http://torch.ch) importer.
+       It allows you to use networks, generated by Torch7 [nn](https://github.com/torch/nn/blob/master/README.md) module.
+
+-# *Generate* the project and build it:
+    - If Makefiles generator on Unix was used, run the following in terminal:
+      @code
+      cd opencv_root/build_opencv
+      make all
+      @endcode
+    - If MS Visual Studio generator was used, then open OpenCV.sln from **opencv_root**/build_opencv folder in the Visual Studio, and build it by using F7 shortcut
+
+-# Now you can build own program by using libraries were built into **opencv_root**/build_opencv/lib.
+   See the following tutorials to learn how to create a program using OpenCV:
+       - @ref tutorial_windows_install
+       - @ref tutorial_linux_gcc_cmake