enumerate build/publish/release scenarios

[SethTisue]

Jenkins job	bootstrap?	publish artifacts to	upload release bundles to	version number format
validate-main	no	scala-ci... scala-pr-validation-snapshots	n/a	2.11.7-0047e5c-SNAPSHOT
release-main	yes	(*) sonatype...snapshots	scala-lang.org/.../nightly	2.11.8-a0dabe3-nightly
release-main	yes	sonatype...releases	downloads.typesafe.com	2.11.8, 2.12.0-M3, etc

(*) = not implemented yet

[SethTisue]

I don't really remember what this was about, and it's changed anyway