Add CONTRIBUTING.md, mainly to document the "next" vs "master" branching
Add CONTRIBUTING.md, mainly to document the "next" vs "master" branching

This file is displayed to would-be pull-request submitters, so it's a good place to provide this information.

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paulbdavis/ledger,jwakely/ledger,jwakely/ledger,paulbdavis/ledger,duncanmortimer/ledger,afh/ledger,jwakely/ledger,duncanmortimer/ledger,afh/ledger,afh/ledger,ledger/ledger,afh/ledger,paulbdavis/ledger,ledger/ledger,ledger/ledger,paulbdavis/ledger,paulbdavis/ledger,duncanmortimer/ledger,paulbdavis/ledger,duncanmortimer/ledger,jwakely/ledger,duncanmortimer/ledger,duncanmortimer/ledger,duncanmortimer/ledger,ledger/ledger,jwakely/ledger,paulbdavis/ledger,jwakely/ledger,jwakely/ledger,ledger/ledger,afh/ledger