Some minor restructuring of the SASS files to reflect the architecture of USWDS

Add a brief README explaining best practices for styling
Add comment annotations to the top of each file that references the original USWDS documentation and source

scss/elements/_tables.scss

@@ -0,0 +1,5 @@
+/*
+ * Tables
+ * @see https://designsystem.digital.gov/components/tables/
+ * @source https://github.com/uswds/uswds/blob/develop/src/stylesheets/elements/_table.scss
+ */