[WIP] docs(cb-barrels): add barrels cookbook

[wardbell]

WIP - DO NOT MERGE
Describes barrels: why we want them and how to create them

Ready for review and copy edits.

@jmcooper - Please do NOT merge or rebase from master. Please leave that to me or someone more expert in git.

This is a squashed and rebased copy of Jim's original in PR #1419
googlebot: it's OK. Jim is in on the gag.

[googlebot]

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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[jmcooper]

Thanks, @wardbell. Sorry for the merge headaches.