Update docs referencing referencing /src files (#291)

* Remove text referencing /src files

* Add link to stack reconciler on 15-stable branch

Author: ksenna

content/docs/implementation-notes.md

@@ -14,7 +14,7 @@ It is very technical and assumes a strong understanding of React public API as w
 
 It also assumes an understanding of the [differences between React components, their instances, and elements](/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.
+The stack reconciler was used in React 15 and earlier. It is located at [src/renderers/shared/stack/reconciler](https://github.com/facebook/react/tree/15-stable/src/renderers/shared/stack/reconciler).
 
 ### Video: Building React from Scratch