Create RPM spec file for Fedora packaging

[miabbott]

Create a tar-diff.spec file following Fedora Go Packaging Guidelines for submission to Fedora.

Tasks

• Create tar-diff.spec following https://docs.fedoraproject.org/en-US/packaging-guidelines/Golang/
• Include main package with CLI tools (tar-diff, tar-patch)
• Include devel subpackage for Go library
• Include man pages
• Create PACKAGING.md documenting the build and packaging process for distribution packagers

Acceptance Criteria

• Spec file passes rpmlint
• rpmbuild produces valid SRPM and RPM
• Package installs correctly with dnf
• PACKAGING.md provides clear instructions for packagers

Dependencies

• Depends on Add make dist target for source tarball creation #19 (make dist target)
• Depends on Create man pages for tar-diff and tar-patch #40 (man pages)