Add instructions for signing the Chips Alliance CLA

Use sbt-sriracha for fully SBT-managed source dependencies. They are
also toggleable via JVM System Properties: sbt.sourcemode and
sbt.workspace.

Update Makefiles and build rocketchip fat jar.

Support switching between source and published chisel3 and firrtl, test
in CI. Document this flow for bumping and describe impact of this flow
on IDE use.

Mainly to mention that chisel3 and firrtl should only be bumped to
stable branches

Co-authored-by: Megan Wachs <megan@sifive.com>