Change license to BSD 3-Clause, on the basis of:

- Riccardo: "I've spoken to GloBee, and they are fine with the licensing, as long as it's liberal (BSD 3-clause / MIT preferred)."
- Also have confirmation from Rehrar that he's happy with permissive licensing.
On that basis, am proposing to change the license to BSD 3-Clause, which is the same licence that the rest of the Monero site uses.
There wasn't an intentional decision at the start that this work should be copyrighted, but it was, and now we can fix that.