Update the README to make it clear Braindrop isn't a "thin client"

[davep]

With #94 in mind (a perfectly reasonable and valid observation) I should update the README to make it very clear that Braindrop isn't designed to be a thin client for all of the Raindrop API, it isn't designed as a one-service terminal-based browser of sorts. Instead it's designed as a "slurp down all the data and work on it locally" client.

As such it might not be the tool for people with a huge collection of raindrops.