stub out a contributing doc

CONTRIBUTING.md

@@ -0,0 +1,9 @@
+# Updating compile-fail test outputs
+
+`bitflags` uses the `trybuild` crate to integration test its macros. Since Rust error messages change frequently enough that `nightly` builds produce spurious failures, we only check the compiler output in `beta` builds. If you run:
+
+```
+TRYBUILD=overwrite cargo +beta test --all
+```
+
+it will run the tests and update the `trybuild` output files.