struggled to resolve dependencies, finally done

I struggled a lot trying to resolve the dependencies when running sbt in the rocket-chip directory. Hopefully you can improve the README to save others' time :)

This is what I did:

(1) Create ~/.sbt/repositories file as:
[repositories]
local
aliyun: http://maven.aliyun.com/nexus/content/groups/public/
sbt-plugins-repo: http://repo.scala-sbt.org/scalasbt/sbt-plugin-releases/, [organization]/[module]/(scala_[scalaVersion]/)(sbt_[sbtVersion]/)[revision]/[type]s/\[artifact\](-[classifier]).[ext]
typesafe-sbt-plugins: http://dl.bintray.com/sbt/sbt-plugin-releases/, [organization]/[module]/(scala_[scalaVersion]/)(sbt_[sbtVersion]/)[revision]/[type]s/\[artifact\](-[classifier]).[ext]
central: http://repo1.maven.org/maven2/

(2) Add these two options to /usr/local/etc/sbtopts (on Mac, sbt installed by brew, YMMV):
-Dsbt.override.build.repos=true
-Dsbt.repository.config=~/.sbt/repositories

Start with the README covering setup in the rocket-chip directory and compare its dependency instructions with the reported sbt workflow. Check whether ~/.sbt/repositories and /usr/local/etc/sbtopts are still relevant, then update the README so a fresh setup can resolve dependencies without the reported struggle.

Written by the indexing model from the issue text.