compile v3.3 on Linux, dotnet 9

[tissatussa]

i managed to compile on Xubuntu 22.04 and the x64-86 binary runs fine in CuteChess GUI. But using net-9 was not mentioned. I also had trouble setting the proper dotnet version, i have several installed on my OS. Please update the README. Btw. your coding seems primary for Windows, am i right ?