feat: update README
feat: update CONTRIBUTING
fix: add note for Linux
fix: rename xdg shortcut
fix: remove snap
fix: rename AyuGram executable in the updater
fix: use different GUIDs
(& rename instructions to the actual state)
Old instruction names were from the time when tdesktop used GYP that had MSVC, XCode and CMake backends.
But tdesktop uses CMake now, so these names are no longer correct.