Explain better how to add a NPM package to a theme/package

[juanmaguitar]

Related to this:

• https://community.frontity.org/t/detect-node-modules-under-packages-folder-remove-them-and-launch-reinstall-from-root/4238
• https://community.frontity.org/t/i-am-started-getting-error-typeerror-cannot-read-property-add-of-undefined/1975/18?u=juanma

[juanmaguitar]

This explanation should be covered as part of the packages sub-section planned as part the section "Core Concepts" (as part of the Learning Curriculum implementation) #265