Add note clarifying context hash usage when passing resources through the ResourceSerializer

[kevinzwhuang]

Description:
I found the documentation to be lacking in explaining how context is passed in when using the ResourceSerializer.

Only after playing around with the ResourceSerializer for some time that I found that the second argument for initializing a resource was the context itself. In the documentation, this second argument was simply nil without explanation that this was actually the context the resource initializes with:
JSONAPI::ResourceSerializer.new(PostResource).serialize_to_hash(PostResource.new(post, nil))

Changes Proposed:

• Add a note clarifying how to pass in a context hash when using the ResourceSerializer.
• Add a code example that shows how it works, building on top of the first example, which leads into the results in the ResourceSerializer section below it.

Hopefully this will help newcomers who might need context for their resource while using ResourceSerializers. Hope this helps @lgebhardt!

[lgebhardt]

@kevinzwh Thanks!