Add new walkthrough-style Explainer.md

Author: lukewagner

README.md

@@ -4,10 +4,11 @@ This repository is where the Component Model is being standardized. For a more
 user-focused explanation, take a look at the **[Component Model Documentation]**.
 
 This repository contains:
-* high-level [goals], [use cases], [design choices] and [FAQ] docs
-* low-level [WIT], [text format], [binary format], [linking], [concurrency] and
-  [ABI] docs
+* a general [Explainer] that introduces the Component Model via WAT examples
+* focused explainers covering [linking] and [concurrency]
+* detailed definitions of the [text format], [binary format], [ABI] and [WIT]
 * a growing [WAST test suite]
+* the original high-level [goals], [use cases], [design choices] and [FAQ] docs
 
 In the future, this repository will additionally contain a [formal spec] and a
 reference interpreter.