Add basic CONTRIBUTING.md that points to contribution guide docs (spack#6203)

tgamblin · web-flow · commit 9c774cccdeee · 2017-11-08T11:09:11.000-08:00
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -0,0 +1,5 @@
+# Contributing to Spack
+
+Before contributing to Spack you should read the
+[Contribution Guide](https://spack.readthedocs.io/en/latest/contribution_guide.html),
+which is maintained as part of Spack's documentation.
