Add documentation on how to supports mutiple targets

[MangelMaxime]

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: https://github.com/thoth-org/Thoth.Json/issues/206

Demo repo: https://github.com/MangelMaxime/fable-example-multi-targets