Scc 0.3.10

This PR documents `scc` usage changes since version 0.3.8 .

- add section about `scc update-submodules` command
- add section about `scc unrebased-prs` command including workflow description with the 0.3.10 additions
- expose the `--shallow`/`--remote` options in `scc merge`
- add `--rebased-to/from` comments to the `scc rebase` workflow description

/cc @bpindelski