Create a document explaining Bash version workarounds

[mbland]

Inspired by the need for @go.join and @go.split per #62, #81, and commit 99ab7805e6ef0a14568d8a100eec03bb2cb03631. All these deep technical reasons should not get lost to the sands of commit log history, or excessively clutter the code comments in multiple places, if there's an effective practice focused on maintenance of a centralized artifact.