Clarify source code license

[AbigailBuccaneer]

I'd like to add Bootil to a package manager (in order to use it as a dependency for gmad, in order to automate building of Workshop addons, ...), and that requires listing the source code license. The only license info with this repository is "feel free to steal :)". Is that supposed to imply that it's public domain, or just redistributable without attribution?

[sr229]

We should opt in to MIT since its permissive enough for Bootil