Added contributing guidelines (wip).

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+## Requirements
+
+Requirements to build and run the unit tests:
+- CMake 2.8 or later
+- GCC / Clang with C++11 support
+
+## Setup
+
+Pull Google Test / Google Mock subrepository:
+```
+$ git init submodules
+```
+
+Create build directory, run CMake, build and run unit tests:
+```
+$ mkdir build && cd build
+$ cmake ..
+$ make
+$ ctest --verbose
+```