Adds a reference to using leveldown-prebuilt as a replacement for leveldown (Windows)

I've explored some *down libraries as replacement for leveldown:

 * sqldown
 * medeadown
 * locket

None of those reachs the performance of leveldown, OR their integrations is missing/buggy (medeadown) so i've though on adding this text to the Common Errors page, specially for Windows users.