"make dist": Helpful error message if not using GNU tar

Only GNU tar supports --transform to modify the paths of the
files in the archive. On a BSD system install GNU tar and run
"make dist TAR=gnutar".
Instructions for OpenSUSE Build Service

We use the OpenSUSE Build Service to build packages for various systems
(currently only Fedora 16 and 17 (x86 and x86_64).

Obviously publishing a new package requires knowing the password to the
stb-tester account on the OpenSUSE Build Service. I am documenting the
procedure for my own benefit, and that of any future maintainers.
README: Add "and contributors" to Authors

Now that we have received a couple of patches from external
contributors. See the git log for details.