Step missing from readme

At the step "npm run dev" I got the error "sh: dxp-next: command not found".

Googling brought me to https://docs.squiz.net/squiz-dxp/latest/dxp-cli/get-started/installation.html and doing 

npm install --global @squiz/dxp-cli-next

fixed it, but that should really be included in the readme here.

