Document isMainModule and appType for libraries

[yglukhov]

# foo.nim
when isMainModule: echo "hi from foo"

# bar.nim
import dynlib
when isMainModule: echo "hi from bar"
doAssert(not loadLib("./libfoo.so").isNil)

nim c --app:lib foo
nim c -r bar

Output:

hi from bar
hi from foo

Expected output:

hi from bar

Note: I'm not sure it's an issue, but I'm getting a lot of questions about it in nimpy.

[Araq]

Works as expected. We can improve the documentation and mention appType == "lib" and similar.

[ringabout]

@Araq Can you explain how to use appType == "lib" to make the example work?

[ghost]

@xflywind when isMainModule and appType != "lib" I guess