Create howToBuild.md

doc/howToBuild.md

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+# How to build PhasicFlow? 
+PhasicFlow uses CMake as the build system. you need to have CMake-3.22 or higher installed on your machine. you can either use the command line for setting-up the build system, or use `cmake-gui` to setup your build system through the GUI. 
+### Step 1:
+Make sure you have installed all the required packages on your computer (tbb and CMake and compilers). 
+### Step 2: Cloning Kokkos
+It is assumed that Kokkos source is located in the home folder of your OS. Clone the current version of Kokkos into your home folder:
+
+```
+cd ~
+mkdir Kokkos
+cd Kokkos
+git clone https://github.com/kokkos/kokkos.git
+```
+
+or simply download and extract the source code of Kokkos in `~/Kokkos` folder. In the end, the top level CMakeLists.txt file should be located in `~/Kokkos/kokkos` folder. 
+
+### Step 3: Cloning PhasicFlow
+Create the PhasicFlow folder in your home folder and then clone the source code into that folder:
+
+```
+cd ~
+mkdir PhasicFlow
+cd PhasicFlow
+git clone https://github.com/hamidrezanorouzi/phasicFlow.git
+```
+### Step 4: Environmental variables
+In the terminal enter the following command:
+
+`gedit ~/.bashrc`
+
+and add the following line to the end of file, save and close it.
+
+source $HOME/PhasicFlow/phasicFlow/cmake/bashrc
+
+close the terminal. 
+
+### Step 5: Building PhasicFlow
+Follow one of the followings to build PhasicFlow for one mode of execution.
+#### Serial build for CPU
+Open a new terminal and enter the following commands:
+```
+cd ~/PhasicFlow/phasicFlow/build
+cmake ../ -DpFlow_Build_Serial=On
+make install
+```
+For faster builds, use `make install -j`. This will use all the CPU cores on your computer for building. 
+#### OpenMP build for CPU
+ 
+#### GPU build for parallel execution on CUDA-enabled GPUs
+
+
+
+After building, `bin`, `include`, and `lib` folders will be created in `~/PhasicFlow/phasicFlow/` folder. Now you are ready to use PhasicFlow.