github: add an issue template

Add an issue template to encourage people to ask questions on Stack Overflow.

.github/ISSUE_TEMPLATE

@@ -0,0 +1,17 @@
+You are opening a _bug report_ against the LibGit2Sharp project: we
+use GitHub Issues for tracking bug reports and feature requests.  If
+you have a question about an API or usage, please ask on StackOverflow:
+http://stackoverflow.com/questions/tagged/libgit2sharp.
+
+Otherwise, to report a bug, please fill out the reproduction steps
+(below) and delete these introductory paragraphs.  Thanks!
+
+### Reproduction steps
+
+### Expected behavior
+
+### Actual behavior
+
+### Version of LibGit2Sharp (release number or SHA1)
+
+### Operating system(s) tested; .NET runtime tested