modfile: add package documentation

Fixes golang/go#36757

Change-Id: I674edfabed7f4bb54c909eda56ca4a87b037fe83
Reviewed-on: https://go-review.googlesource.com/c/mod/+/216842
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
2 files changed