I'm not sure when Adobe instituted this in Lightroom, but they've included a reference cheat sheet for shortcuts as a shortcut. Press Ctrl + / (Command + / on Macs) and you'll get the above screen with all of the shortcuts for the module you're in. Click anywhere on the screen and it will disappear, returning you to edit mode.
One of my favorite time-savers isn't on the sheet though-- we'll not precisely. I rate my photos in order to decide which I'll be editing from a session. Typically, I'll make two passes: the first two weed out the worst; the second to identify the best. You can rate your images step-wise using the left and right brackets, or by using your number pad. Hold down the Shift key while numbering, and Lightroom will automatically advance to the next photo. To make life even simpler, hit the Cap Lock and you don't have to hold down the Shift key. It's a time saver that adds up over 200 photos.
Other great time savers:
- K = Adjustment Brush
- R = Crop
- M = Gradual Filter