Add design doc about optionality

Many patches these days ignore the concept of features being optional,
and measuring impact to the project. This documentation attempts to
close that gap, by having

Add design doc about optionality

Many patches these days ignore the concept of features being optional,
and measuring impact to the project. This documentation attempts to
close that gap, by having some explicitly defined language in the
architecture that developers can reference in their commit messages, and
maintainers can point to for general guidance on option flags.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Id1ab8f1765110f58f22670db18db7ed226da6b57

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