docs: Added OpenCV as requirement

When running the installation it required OpenCV to be installed.

Author: justinmerrell

README.md

@@ -11,6 +11,7 @@ Requirements
 ============
 * pcompiler  (Use_qt4 branch: https://github.com/kipr/pcompiler/tree/use_Qt4)
 * CMake 2.6.0 or later
+* OpenCV
 * [Qt >= 4.7.4](https://www.qt.io/download-qt-installer)