Document sharing of library dependencies

[gzm0]

Here: vmunier/play-with-scalajs-example#20

[ochrons]

Is this now obsolete?

[gzm0]

Self-assigning for verification.

[sjrd]

Is it what's covered in the last section of http://www.scala-js.org/doc/project/cross-build.html ?

[gzm0]

Not really... This was about being able to factor dependencies out.

E.g.:

val fooBarSettings = Seq(
  libraryDependencies ++= Seq(a,b,c)
)

Instead of (following does not work it there is a %%% macro in any of a,b,c):

val fooBarLibs = Seq(a,b,c)