_content/ref/mod: document special case for LICENSE
_content/ref/mod: document special case for LICENSE

Change-Id: Ie15dc10304d430d1fb3201931fc5c161581f79ea
Reviewed-on: https://go-review.googlesource.com/c/website/+/340209
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website