Add notes about why AppImages are good

[pfirsich]

Notes to write out later:

• Sometimes troublesome to start old löve games
• The löve in your package repository might use a newer LuaJIT version or one that's compiled with Lua 5.2 compat, which breaks some Lua functions. With an AppImage you would notice that your game does not run right away.

[hollunder]

• Distribute a single file, about as easy as it gets for a user to run.
• No need to install anything. (I definitely needed to install something one some systems, FUSE I think, but not sure)

[pfirsich]

Good points, thank you!