Describe the building procedure

[vmarkovtsev]

As promised in https://github.com/saiarcot895/chromium-ubuntu-build/issues/91#issuecomment-685439389

[lorenz]

Just FYI this is what I'm using in my CI (it's a bit shorter than what's proposed here, also based on a clean 20.04 container):

apt-get update && apt-get install --no-install-recommends -yy make wget xz-utils devscripts equivs quilt
cp debian/changelog.focal debian/changelog
cp debian/control.standard.focal debian/control
export ORIG_VERSION=$(head -1 debian/changelog.focal | grep -oE '([0-9]+\.){3}[0-9]+')
debian/rules get-packaged-orig-source
tar -xf chromium-browser_$ORIG_VERSION.orig.tar.xz --strip-components=1
QUILT_PATCHES=debian/patches quilt push -a
mk-build-deps -t 'apt-get --no-install-recommends -yy' --install debian/control
sed -i '0,/<list type="fd" name="buffers">/s//<list type="fd" name="buf">/' /usr/share/xcb/dri3.xml
debuild -b -uc -us