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

`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
```