Update readme

Document required command line options
Incorporate offset into calculation of positions clicked
Make sure offset is calculated using the same width and height as terrain
Fix error in rendering order
Load cliff faces
Simple rendering of cliffs - still need to deal with clipping of sloped cliffs
Dedupe some code
Rename width/height to xMax/yMax
Start work on sloped cliffs - still need to do diagonal clipping
Implement diagonal clipping
Load in the full set of cliff sprites
Store all cliff sprites in a single list
Tidy up cliff rendering
Load tracks
Move code for loading object references into DatFileInputStream
Make sure compatible vehicles are loaded in
Make terrain flat by default
Extract a renderer interface
Create a simple track rendering demo
Add a SpriteLayer enum so we can draw junctions correctly