readme: add note about GLAD_ prefixed symbols in C
readme: add note about GLAD_ prefixed symbols in C

dbralir/glad,dbralir/glad,valeriog-crytek/glad,0x1100/glad,QUSpilPrgm/glad,bsmr-opengl/glad,aaronmjacobs/glad,QUSpilPrgm/glad,valeriog-crytek/glad,aaronmjacobs/glad,valeriog-crytek/glad,hrehfeld/glad,bsmr-opengl/glad,bsmr-opengl/glad,hrehfeld/glad,QUSpilPrgm/glad,dbralir/glad,aaronmjacobs/glad,hrehfeld/glad,valeriog-crytek/glad,0x1100/glad,0x1100/glad,hrehfeld/glad,dbralir/glad,QUSpilPrgm/glad,0x1100/glad,aaronmjacobs/glad,bsmr-opengl/glad