Add note that SHA-1 actually has been broken

[stuartpb]

• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

[stuartpb]

I've adjusted the wording on that first line: "There have been attempts to create a synthetic collision attack" makes it sound like there've only been attempts, but no actual collisions. That was the case, when this book was written, but as of 2017, there have been successful attempts, at great expense, which is the point I'm trying to illustrate here: while it is possible to have two files with the same SHA-1 hash, it is something you have to work very hard and intentionally to cause.

[ben]

I think we've got it! Thanks for the contribution!