Add documentation on how to supports mutiple targets #201
Description
If there are no target specific dependencies then a single package with compiler directives can works
If there are different dependencies depending on the target, then using multiple projects is the recommended way to go.
Past discussions: thoth-org/Thoth.Json#206
Demo repo: https://github.com/MangelMaxime/fable-example-multi-targets