Add an example that covers this:from="this" to can-slot’s docs

[chasenlehara]

can-slot’s docs cover passing context to a template, but they don’t mention that you can pass the template’s entire context to the slot by writing this:from="this", as mentioned in this comment: #198 (comment)

<can-slot name="test" this:from="this">
    <a-component object:from="myObject" />
    <!-- myObject is defined outside the can-slot -->
</can-slot>