Document method for getting 'sbt' both inside and outside of Google

[TCal]

sbt is needed to rebuild VexRiscv
(see https://cfu-playground.readthedocs.io/en/latest/projects/z_customizing_vex.html).

These are some alternate instructions for acquiring sbt, but I'm not sure if this is appropriate, and there may be Java dependencies that aren't satisfied:

mkdir ~/bin && cd ~/bin
wget https://raw.github.com/paulp/sbt-extras/master/sbt
chmod +x sbt

[Alan Green]

https://sdkman.io/install looks convenient but a little scary. It support sbt and scala: https://sdkman.io/sdks#sbt

[TCal]

There's also https://github.com/sbt/sbt. I haven't tried it.