Add SBT uber-jar creation info

[AlexDBlack]

Something that some SBT users run into is uber-jars that give a Nd4jBackend.NoAvailableBackendException, due to incorrect packaging with plugins like sbt-assembly plugin (for example, missing or not adding java service loader files under /META-INF/services/

Having something we can point users to for this would be useful.

Due Date

To be completed by: YYYY-MM-DD

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[AlexDBlack]

From gitter: Cause for one user was discard merge strategy removing all of the service loader files:

assemblyMergeStrategy in assembly := {
    case PathList("META-INF", xs @ _*) => MergeStrategy.discard
    case x => MergeStrategy.first
}   

Fixed by:

assemblyMergeStrategy in assembly := {
    case PathList("META-INF", "MANIFEST.MF") => MergeStrategy.discard
    case x => MergeStrategy.first
}

(Note that this discards the manifest file, which is probably OK in most cases, but not all)

[crockpotveggies]

@AlexDBlack where are you thinking we should put this? In this page? https://deeplearning4j.org/docs/latest/deeplearning4j-config-buildtools

[barrybecker4]

Thanks. The above fix worked for me. I was struggling with this for quite a while.
I saw it using sbt 1.3.8, sbt-assembly 0.14.10, and nd4j 1.0.0-beta3 and 1.0.0-beta6