This is a duplicate of Generate scaladoc for root package, however the Answer does not state where sbt doc looks for the rootdoc.txt.
I added
scalacOptions in doc ++= Seq("-doc-root-content", "rootdoc.txt")
to my build.sbt, but sbt doc does not seem to scan it. I tried to put it next to the build.sbt, in src, src/main, src/main/scala
I am using sbt 0.12.3
It seems that your arguments are not given to scaladocat all. I cannot figure out why the command line arguments are not passed when scoping to doc, but it works if you do not scope it to docbut to Compile:
scalacOptions in Compile ++= Seq("-doc-root-content", "rootdoc.txt")
With rootdoc.txtat the root of your project.