Add component variables to component readme.md

[slkennedy]

For each readme there should be a section under the sub-element documentation that identifies the variables used to style that element. This is a longer/larger task because you'll have to go through every component to finish this one.

Example from Willow Banner

willow-banner__icon

• Optional
• Restrictions
  • Should Contain: Nothing

Variables Used

• $component-banner-icon-height
• $component-banner-icon-width

Notes

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