Add Code Climate badge

Per discussion with @dmethvin, this adds a link (via a badge) to Code Climate from the README, in order to provide easy access to jQuery's code metrics to contributors.

For those who may be unfamiliar, Code Climate is a free-for-OSS tool that helps developers track the quality of their projects over time. We use static analysis to detect possible problem spots in a codebase and then provide feedback so the issues can be reviewed and potentially addressed earlier in the dev cycle.