Add use of Flake8 to contributing.txt
Add use of Flake8 to contributing.txt

pexip/meson,aaronp24/meson,trhd/meson,aaronp24/meson,QuLogic/meson,aaronp24/meson,QuLogic/meson,jpakkane/meson,aaronp24/meson,fmuellner/meson,trhd/meson,trhd/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,thiblahute/meson,thiblahute/meson,thiblahute/meson,becm/meson,becm/meson,aaronp24/meson,mesonbuild/meson,becm/meson,wberrier/meson,QuLogic/meson,trhd/meson,MathieuDuponchelle/meson,jpakkane/meson,jeandet/meson,pexip/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,ernestask/meson,ernestask/meson,QuLogic/meson,becm/meson,jeandet/meson,jpakkane/meson,fmuellner/meson,mesonbuild/meson,fmuellner/meson,jpakkane/meson,wberrier/meson,mesonbuild/meson,aaronp24/meson,pexip/meson,becm/meson,thiblahute/meson,mesonbuild/meson,jeandet/meson,fmuellner/meson,ernestask/meson,MathieuDuponchelle/meson,wberrier/meson,MathieuDuponchelle/meson,mesonbuild/meson,trhd/meson,jeandet/meson,fmuellner/meson,thiblahute/meson,wberrier/meson,aaronp24/meson,trhd/meson,thiblahute/meson,ernestask/meson,QuLogic/meson,becm/meson,MathieuDuponchelle/meson,trhd/meson,ernestask/meson,MathieuDuponchelle/meson,fmuellner/meson,pexip/meson,pexip/meson,pexip/meson,QuLogic/meson,ernestask/meson,wberrier/meson,becm/meson,becm/meson,wberrier/meson,becm/meson,wberrier/meson,jeandet/meson,fmuellner/meson,pexip/meson,fmuellner/meson,QuLogic/meson,thiblahute/meson,mesonbuild/meson,aaronp24/meson,MathieuDuponchelle/meson,wberrier/meson,thiblahute/meson,jeandet/meson,QuLogic/meson,jeandet/meson,trhd/meson,mesonbuild/meson,jeandet/meson,fmuellner/meson,ernestask/meson,jpakkane/meson,jpakkane/meson,pexip/meson,jpakkane/meson,aaronp24/meson,jeandet/meson,becm/meson,ernestask/meson,trhd/meson,ernestask/meson,thiblahute/meson,mesonbuild/meson,pexip/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson