Add note about Go Unicode character classes

As proposed on golang-nuts:
  https://groups.google.com/g/golang-nuts/c/M3lmSUptExQ/m/hRySV9GsCAAJ

See also:
  https://go-review.googlesource.com/c/go/+/264678

Change-Id: I028dd1abccfd5372f31e21c3453fc9a40cc343ad
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58030
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed