Style guide: Add section about switch

[yupinghu]

See comment on #73 re: switch and add that to the style guide.

[yupinghu]

Namely:

Style and assert/error on default fall-through.

    switch (req.query.action) {
        case 'subscribe': {
            return onSubscribe(req, res, next);
        } break;
        case 'unsubscribe': { 
            return onUnsubscribe(req, res, next);
        } break;
        default: {
            assert(false, 'some descriptive message here');
        } break;
    }

[thmiceli]

Is the 'break' statement on the same line as the closing curly brace a common pattern? It seems odd to me. It is also deadcode after a return statement, that doesn't bother me as a reader of the code but it may be reported by static analysis tools.

[ericvw]

For background/motivation, the convention ensures that the scope block encompass all statements related to the case. It is arbitrary for the break; to be on the same line as the closing }, but it makes some sense because you really want to end the case for that scope block.

Since break; would be a no-op after a return, should static code analysis tools ignore that?

[yupinghu]

I think Thierry's point is precisely that the break is unreachable because of the return, hence it'd be reported as dead code by a naive checker. :)