Add a brief description on the new vex.bbclass that is used to generate
metadata needed by external tools to check for vulnerabilities.
(From yocto-docs rev: 32cf3414f5ed127f59119e38639bc8dbd57b7891)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>