Scc 0.3.10 (rebased onto develop)



This is the same as gh-472 but rebased onto develop.

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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