Clarify that add-apt-repository depends on software-common-properties
Clarify that add-apt-repository depends on software-common-properties

I got the "command not found" error for add-apt-repository on a clean Ubuntu installation and I think that the docs can be friendlier if they specify this :)

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Reviewed-on: https://github.com/bazelbuild/bazel/pull/691
MOS_MIGRATED_REVID=110062139

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