Add User Support issue template. (#5651)

This adds a new issue template for user support issues. These are
distinct from bug reports because they may not be bugs and might just be
users needing help.

.github/ISSUE_TEMPLATE/support.md

@@ -0,0 +1,28 @@
+---
+name: User support
+about: Ask the maintainers a question or for support.
+title: ''
+labels: ['user-support', 'needs-triage']
+assignees: ''
+
+---
+
+**What is the issue you're encountering?**
+<!--- Please provide a few sentences describing the issue you encountered. --->
+
+**Steps to Reproduce (if applicable)**
+<!--- If you're encountering a problem, please provide a description of how you
+got to the state you're in. If possible please provide source and tool command
+line options. If the issue reproduces on Compiler Explorer
+(https://godbolt.org/) or Shader Playground
+(https://shader-playground.timjones.io/) please provide a link. If the source is
+split across multiple files, please preprocess into a single file using DXC's 
+command line `-P -Fi <path>`.--->
+
+
+**Environment**
+- DXC version <!-- replace with the output of 'dxc --version' -->
+- Host Operating System <!--- Host operating system and version --->
+
+<!--- If this is a build issue please include CMake version, compiler and
+version, and the CMake invocation used to configure your build directory. --->