CONTRIBUTING references CLA, but this appears to have been replaced with DCO

[bh-wise]

The CONTRIBUTING guide references a need to sign the Contributor License Agreement and links to https://cla.pivotal.io/sign/spring. This link does not work as cla.pivotal.io does not have any DNS records. I believe this is because the Spring team have replaced the requirement to sign a Contributor License Agreement with a Developer Certificate of Origin and the DCO GitHub App appears to be enforcing this.

On this basis I think we need to make a small update to the CONTRIBUTING guide, for example:

For all non-trivial patches or pull requests we need every commit to include a "Signed-off-by trailer" confirming that you agree to the Developer Certificate of Origin. See the git documentation if you are unfamiliar with how to do this.

[bh-wise]

Are we happy that this is an accurate summary of the situation? If so I'm happy to put together a small PR.