Add an issue template (dotnet#2686)

.github/issue_template.md

@@ -0,0 +1,46 @@
+### Steps to reproduce
+
+Either include a project sample, attach a zipped project, or provide IDE / CLI steps to create the project and repro the behaviour. Example of a project sample:
+
+Project file
+```xml
+<Project>
+  <PropertyGroup>
+    <Extension>cs</Extension>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <I Include="**/*.$(Extension)"/>
+  </ItemGroup>
+
+  <Target Name="Build">
+    <Message Text="I: %(I.Identity)"/>
+  </Target>
+</Project>
+```
+
+Directory contents:
+```
+/
+- a.cs
+- b.cs
+- dir/
+     - c.cs
+```
+
+Command line
+```
+msbuild /bl
+```
+### Expected  behavior
+
+
+### Actual behavior
+
+
+### Environment data
+`msbuild /version` output:
+
+OS info:
+
+If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):