Document the possible need to install shellcheck

[arvoelke]

In order to run the static script locally I needed to first sudo apt-get install shellcheck (on Ubuntu 18.04.3 LTS). This was a little confusing to me as I thought this was a missing pip package, while I had already tried .ci/static.sh install and couldn't find any mention of spellcheck in the doc search.

A good place to document this might be on the intro page under a new section titled something like "Local usage". This new section would be a good place to include some commonly-used commands such as:

bones-generate --output-dir .ci ci-scripts && .ci/static.sh script