Clear instructions for running on Linux

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.

Start by reproducing the listed Ubuntu 20.04 setup and inspect NLog.config, SharpFlame/Data/3.1_b4-objects, and SharpFlame/Data/tilesets. Update the Linux instructions with the required package, configuration, directory selection, tile-panel refresh workaround, and Place Objects limitation. Done means a user can follow the documented setup without the reported unclear startup failures.

Written by the indexing model from the issue text.