Amended implementation-notes.md with link to Dan Abramov's post describing difference between React components, elements, and instances (#9388)

* Amended implementation-notes.md to include a link to a blog post by Dan Abramov, explaining the difference between components, elements, and instances. An understanding of this distinction is crucial in tracing through Implementation pseudocode, and reading Dan's blog first may ease newcomers into understanding the implementation.

* adjusted wording to maintain stylistic consistency with rest of content, per @aweary's request

contributing/implementation-notes.md

@@ -11,6 +11,8 @@ This section is a collection of implementation notes for the [stack reconciler](
 
 It is very technical and assumes a strong understanding of React public API as well as how it's divided into core, renderers, and the reconciler. If you're not very familiar with the React codebase, read [the codebase overview](/react/contributing/codebase-overview.html) first.
 
+It also assumes an understanding of the [differences between React components, their instances, and elements](/react/blog/2015/12/18/react-components-elements-and-instances.html).
+
 The stack reconciler is powering all the React production code today. It is located in [`src/renderers/shared/stack/reconciler`](https://github.com/facebook/react/tree/master/src/renderers/shared/stack) and is used by both React DOM and React Native.
 
 ### Video: Building React from Scratch