Add basic contributing guidelines

A word-for-word of the README's "Communication" section, for now, but
hopefully it'll steer more people looking for general help toward Stack
Overflow.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>

Author: stephencelis

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# Contributing
+
+ - Need **help** or have a **general question**? [Ask on Stack
+   Overflow][] (tag `sqlite.swift`).
+ - Found a **bug** or have a **feature request**? [Open an issue][].
+ - Want to **contribute**? [Submit a pull request][].
+
+[Ask on Stack Overflow]: http://stackoverflow.com/questions/tagged/sqlite.swift
+[Open an issue]: https://github.com/stephencelis/SQLite.swift/issues/new
+[Submit a pull request]: https://github.com/stephencelis/SQLite.swift/fork