Add fontmake build scripts

[eliheuer]

Scripts for building with fontmake and updating the Google Fonts repo. I have been running them both at the same time from the root directory, like this:

sources/scripts/build.sh && sources/scripts/pull-request.sh

new file:   sources/scripts/build.sh
            This is a basic build script for fontmake
            Run from the root directory

new file:   sources/scripts/pull-request.sh
            This script prepares a PR to Google Fonts
            Run from the root directory after running build.sh

Thanks! I can make a PR adding build instructions to the README or docs if you want.