Ask engineers to help triage issues when they have time

Merged Nick Thomas requested to merge nick.thomas/www-gitlab-com:exhort-people-to-triage into master

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/17693#note_13886635

Ask engineers to consider spending some time triaging issues.

This MR also extends the preamble to include some context about why these issues are coming in, and softens the "enforcement" wording as it came across as being quite forbidding.