Clear instructions for running on Linux

[20kdc]

This issue is more of a way of informing people than an actual issue, but it does highlight some actual issues.

Current instructions on Ubuntu 20.04 LTS:

1. Ensure you have gtk-sharp2 or you will get an unclear exception: System.InvalidOperationException: Could not detect platform. Are you missing a platform assembly?
2. NLog.config must be deliberately modified to have no targets and no rules (i.e. the targets and rules tags must have no contents) or else you will get: System.TypeInitializationException: The type initializer for 'SharpFlame.SharpFlameApplication' threw an exception. ---> System.TypeInitializationException: The type initializer for 'SharpFlame.Core.Result' threw an exception. ---> NLog.NLogConfigurationException: Error when setting property 'Name' on Console Target[(unnamed)] ---> System.NotSupportedException: Cannot invoke method with stack pointers via reflection
3. The object data directory SharpFlame/Data/3.1_b4-objects and the tilesets directory SharpFlame/Data/tilesets must be added. Note that selecting the wrong directory will crash SharpFlame.
4. Scrolling does not refresh the contents of the tile panel. Toggle "Display Tile Types" or such to properly refresh it.
5. I am not sure why the "Place Objects" panel shows no objects.