You can use \xRightarrow[below]{above} from the mathtools (or extpfeil) package or \xLongrightarrow[below]{above} from extarrows. (Have a look at “How to look up a math symbol?” for ideas how you can easily find a particular symbol.)
If you want to keep the the same arrow length as implies, you can use \underset and \overset from amsmath, e.g. \underset{below}{\implies}, or maybe \underset{\mathclap{below}}{\implies} (\mathclap makes it so that “below” doesn’t introduce extra vertical space if it is longer than the arrow. It is included in the immensely useful mathtools package).
There is no need for \substack in this case: there are the \varinjlim and \varprojlim macros taking care of both direct and inverse limits, putting a subscript now results in the behaviour you're looking for (and everyone is mimicking using \substack).
Best Answer
assuming
amsmath
I like to make the arrow slightly longer using, say,