Update README.md to link to the spec doc (#443)

Also add a note pointing out that the details in Overview.md are out of date.

Author: tlively

README.md

@@ -6,15 +6,13 @@
 This repository is a clone of [github.com/WebAssembly/spec/](https://github.com/WebAssembly/spec/).
 It is meant for discussion, prototype specification and implementation of a proposal to add garbage collection (GC) support to WebAssembly.
 
-* See the [overview](proposals/gc/Overview.md) for a summary and rationale of the proposal.
+* See the [overview](proposals/gc/Overview.md) for a high-level summary and rationale of the proposal. *Note:* the concrete details here are out of date.
 
-* See the [MVP](proposals/gc/MVP.md) for a draft specification of the concrete language extensions that are proposed for the first stage of GC support in Wasm.
+* See the [MVP](proposals/gc/MVP.md) for an up-to-date overview of the concrete language extensions that are proposed for the first stage of GC support in Wasm.
 
-* See the [Post-MVP](proposals/gc/Post-MVP.md) for possible future extensions in later stages.
+* See the [Post-MVP](proposals/gc/Post-MVP.md) for possible future extensions in later stages. 
 
-<!--
-* See the [modified spec](https://webassembly.github.io/gc/core) for details.
--->
+* See the [modified spec](https://webassembly.github.io/gc/core) for the completed spec for the first-stage proposal described in MVP.md.
 
 This repository is based on the [function references proposal](proposals/function-references/Overview.md) as a baseline and includes all respective changes.