Small clarification needed in README

[cyberkarim]

Hello guys,

The documentation states that in order to build the engine by compiling g++, you need to use the "make" command line. However, the makefile script uses clang++ as a default compiler. Thus, I wasn't able to compile at first since I didn't have clang++.

Precising CXX=g++ as a parameter for the "make" did it for me.