hippotatd: add a startup message

[hippotat] / CONTRIBUTING

Developer's Certificate of Origin
---------------------------------

All commits to the Hippotat code base must include the line
`Signed-off-by: name <email>' at the end of the commit message.
This indicates that the author certifies the patch under the
`Developer's Certificate of Origin':

  Developer's Certificate of Origin 1.1

  By making a contribution to this project, I certify that:

  (a) The contribution was created in whole or in part by me and I
      have the right to submit it under the open source license
      indicated in the file; or

  (b) The contribution is based upon previous work that, to the best
      of my knowledge, is covered under an appropriate open source
      license and I have the right under that license to submit that
      work with modifications, whether created in whole or in part
      by me, under the same open source license (unless I am
      permitted to submit under a different license), as indicated
      in the file; or

  (c) The contribution was provided directly to me by some other
      person who certified (a), (b) or (c) and I have not modified
      it.

  (d) I understand and agree that this project and the contribution
      are public and that a record of the contribution (including all
      personal information I submit with it, including my sign-off) is
      maintained indefinitely and may be redistributed consistent with
      this project or the open source license(s) involved.