Add the coverage issue to the user guide doc
Add the coverage issue to the user guide doc

Change-Id: Ifdb0620fb6f67ef04044f02b497cc5dbd28a2a6a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3130003
Commit-Queue: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/recipes-py,luci/recipes-py