Add a license to the convert.sh script

[pserwylo]

Could you please add a license to convert.sh, as I'd like to spruce it up and make it more general to be used by other projects.

Longer version: I was pointed towards your convert.sh by @cstiens last year for some work we were doing on F-Droid. We are now doing some more UI stuff, and I again need to take .svg files and convert them into various drawable-* .png files. I've made some changes to the script to make it more general, so that it can be used by any project, but am unable to publish it until I know what license it is under (if any).

Cheers.