Update README to specify correct yarn

[josephwb]

Perhaps it was just me, but when I typed yarn Ubuntu suggested:

Command 'yarn' not found, but can be installed with:

sudo apt install cmdtest

which, of course, is not what is needed. This post gives nice simple instructions. Maybe something along these lines could be added to the README?

Thanks for putting this on github ( -_・)

[josephwb]

Erg maybe add a link to sass too?