add new vendor extension to the README.

Author: mattpolzin

README.md

@@ -228,6 +228,8 @@ Unlike what happens when you lookup an individual component using the `lookup()`
 
 Anywhere that a type would have had either a reference or a component, the dereferenced variety will simply have the component. For example, `PathItem` has an array of parameters, each of which is `Either<JSONReference<Parameter>, Parameter>` whereas a `DereferencedPathItem` has an array of `DereferencedParameter`s. The dereferenced variant of each type exposes all the same properties and you can get at the underlying `OpenAPI` type via an `underlying{TypeName}` property. This can make for a much more convenient way to traverse a document because you don't need to check for or look up references anywhere the OpenAPI Specification allows them.
 
+For all dereferenced types except for `JSONSchema`, dereferencing will store a new vendor extension on the dereferenced value to keep track of the Component Object name the value used to be referenced at. This vendor extension is a string value with the `x-component-name` key.
+
 You can take things a step further and resolve the document. Calling `resolved()` on a `DereferencedDocument` will produce a canonical form of an `OpenAPI.Document`. The `ResolvedRoute`s and `ResolvedEndpoint`s that the `ResolvedDocument` exposes collect all relevant information from the whole document into themselves. For example, a `ResolvedEndpoint` knows what servers it can be used on, what path it is located at, and which parameters it supports (even if some of those parameters were defined in an `OpenAPI.Operation` and others were defined in the containing `OpenAPI.PathItem`).
 
 If your end goal is to analyze the OpenAPI Document or generate something entirely new (like code) from it, the `ResolvedDocument` is by far more convenient to traverse and query than the original `OpenAPI.Document`. The downside is, there is not currently support for mutating the `ResolvedDocument` and then turning it back into an `OpenAPI.Document` to encode it.