Add issue template. (GoogleCloudPlatform#641)

This should help us triage issues, for people who don't realize that this repo
contains multiple apis with multiple samples.

Author: jerjou

ISSUE_TEMPLATE.md

@@ -0,0 +1,12 @@
+## In which file did you encounter the issue?
+
+<!-- Please provide the full path to the file, to avoid ambiguity -->
+
+### Did you change the file? If so, how?
+
+<!-- A diff would be helpful; otherwise, a description -->
+
+## Describe the issue
+
+<!-- Please be specific. Copying and pasting your invocation and the entire
+output is often helpful. -->