Correct README, added build and contribution instructions

BUILD.md

+## Compilation instructions
+
+The pre-requisites are:
+
+* CMake 2.8.10 or higher. CMake >= 3.0.0 if you want to build everything.
+* A C++ compiler
+* For advanced examples, C++11/14, OpenMP support, MPI, ISPC, CUDA, OpenCL ...
+
+In order to build the code, follow the typical CMake steps for an
+out-of-source build:
+
+```bash
+mkdir build
+cd build
+cmake ..
+make
+make install # optional
+```

CONTRIBUTE.md

+## How to contribute
+
+Any contributions (new code samples, bug fixes, improvements etc.) are
+warmly welcome. In order to contribute, please follow the standard
+Gitlab workflow:
+
+1. Fork the project into your personal space
+2. Create a feature branch
+3. Work on your contributions
+4. Push the commit(s) to your fork
+5. Submit a merge request to the master branch
+