README.md should describe run commands

[LeeTibbert]

Please consider having the README.md describe the run commands for
cross projects.

It has been a long while since I have used an sbt multi-project. The simple
'''sbt run'' in my mind from just having finished the
sbt-native tutorial did not work.
I was in the midst of figuring out the required commands for sbt-cross the hard way
when I came across the scalajs-cross tutorial and rejoiced. Adapting freely from that tutorial (I think I am OK with copyright/license here)
'''
sbt> barJS/run
sbt> barJVM/run
sbt> barNative/run
'''
This addition would get the README.md closer to a "Getting started: see one, do one" state.
Agreed that post-introductory work requires a deeper understanding of sbt, scala-native, &
scala-cross.

Thank you for considering this suggestion.

Lee

[MasseGuillaume]

Excellent idea.

First Draft: https://github.com/scala-native/sbt-cross-example

[MasseGuillaume]

Thanks for your feedback! More doc improvement here: https://github.com/MasseGuillaume/sbt-cross-project/tree/better-doc#cross-platform-compilation-support-for-sbt

[LeeTibbert]

@MasseGuillaume, thank you for paying attention to this issue.

1. The sbt-cross-example README.md, after the pending PR, looks
   good. Short & sweet yet gives the essential toehold.

2. Masse/Guillaume/.../better-do#cross-platform-compilation-support
   also looks good. I appreciate your leaving the nit "instanciate"
   for me to find.

   Since you kindly proposed this as a draft, a couple of comments
   the right of the decimal point. In the spirit of continuous improvement:

• The header "Installation" could be "Configuration".

• The fact that the section "Directory Structure" exist at all
  is a win. For a quicky, one could create a new "Advanced Topics"
  heading and subsume "Directory Structure" under it.

  To me,it makes the topic less of an orphan and give a good place
  for additional topics. The idea is not try to document now ever
  last detail, but only ones people stumble over. Saves Devo support.

• For a hypothetical "Advanced" section:

  While trying to get uTest working for pending/eventual scala-native support,
  I ran_into/hosed_myself_by_creating 2-out-of-3 (m-out-of-N)
  issues. Agreed that a full dive into the 2-out-of-3 issues should
  be avoided.

  The information from @sjrd in issue Mid-term: sbt-cross might give hints about handling 2-out-of-3 (m-out-of-N) issues. #17 helped me find my way out
  of the maze and is short.

.platformsSettings(JSPlatform, JVMPlatform)(
    libraryDependencies += ...
)

(with the appropriate imports to import JSPlatform and JVMPlatform).

• A line or two of text to say platformsSettings might be used to handle 2-out-of-3
  shared directory issues (by appropriately appending to sbt's unmanagedSources)
  could be added to give new cross-builder's a clue.

I hope some of this is useful, if only to show that someone is
reading the draft. I am glad to look at future drafts as they might
happen.