Add the issue template for guide-related problems

Author: dkhalanskyjb

.github/ISSUE_TEMPLATE/guide.md

@@ -0,0 +1,20 @@
+---
+name: Issue with the Coroutines guide
+about: Problems on https://kotlinlang.org/docs/coroutines-guide.html
+title: ''
+labels: guide
+assignees: ''
+
+---
+
+**Which page?**
+
+Drop a link to a specific page, if this is not about a page missing altogether.
+
+**What can be improved?**
+
+Describe the problematic part. Does it mislead the reader? Is it completely wrong?
+
+**Is this explained better somewhere else?**
+
+Show us the explanation that made it click for you.