Add instructions to build from source

[kemelzaidan]

Hi,

it would be awesome to add some instructions to build it from source. It would allow packagers to create packages for different distributions and add the game to their repository.

Thanks,

Kemel Zaidan

[matinfo]

Look in the Makefile! Work pretty well.

[CodeAsm]

Is this also working for Linux? cause it doesnt seem to want to build in linux. or im missing some default configs?

ERROR: Cannot export project with preset "Linux" due to configuration errors:
No export template found at the expected path:
/home/codeasm/.local/share/godot3/templates/3.5.3.stable/linux_x11_64_debug
No export template found at the expected path:
/home/codeasm/.local/share/godot3/templates/3.5.3.stable/linux_x11_64_release

   at: _fs_changed (editor/editor_node.cpp:806)
ERROR: Project export for preset "Linux" failed.
   at: _fs_changed (editor/editor_node.cpp:822)
make: *** [Makefile:7: linux] Error 1

$ godot3 project.godot

Did kinda work, could run it from there. (F5)