Fixes#240.
To make this easier, I added a script to aid in downloading icons.
Checkout F-Droid/tools/download-material-icon.sh for more details.
The icons are licensed under the CCv4 attribution license, which I
added a shout out to under "License" in the README.md.
This now figures out the root of the fdroidclient project and works there,
rather than just working in the root on the project. So it can be run like
./tools/zip-build.sh or (cd tools && ./zip-build.sh) or whatever.
I also removed the pointless function, since it is only called once in the
script.
