issue templates: add maintainer away notification

Add a new issue template to notify GCM users and contributors of when
maintainers are away and the repo is not being actively monitored. This
template is intended to be used when all maintainers are away and should
be pinned to maximize visibility.

.github/ISSUE_TEMPLATE/maintainer-away.md

@@ -0,0 +1,17 @@
+---
+name: Maintainer Away Notification
+about: Notifies GCM contributors and users of when maintainers are away (e.g.
+for vacation, family leave, etc.).
+title: 'GCM maintainer(s) will be away from: <Begin Date>-<End Date>'
+labels: ''
+assignees: ''
+---
+
+**Maintainer(s) are away**
+
+The GCM maintainer(s) will be away on the following dates:
+
+<Begin Date>-<End Date>
+
+During this time, the maintainer(s) will not be actively monitoring issues, PRs,
+discussions, etc.