Add a Contributing file with some Bash guidelines

Author: ingydotnet

.github/Contributing.md

@@ -0,0 +1,17 @@
+## Bash Style Conventions
+
+* Use spaces for indentation.
+  No tabs please.
+* Use 4 space indent.
+* Keep lines under 80 chars if possible.
+* Always use `[[ ... ]]`, not `[ ... ]` form.
+* Don't use `-n`.
+* Don't quote things that don't need quotes.
+  * Unless it really adds to readability.
+* If script is long, break into functions and use a `main` function at top.
+* Function names should be `foo-bar` not `foo_bar`.
+* Define functions `foo-bar() ( ... )`, not `function foo-bar ( ... )`.
+
+## Testing
+
+* Run `make test`.