chore: update readme [ci skip]

Author: yyx990803

README.md

@@ -1,32 +1,18 @@
 # vue-next [![CircleCI](https://circleci.com/gh/vuejs/vue-next.svg?style=svg&circle-token=fb883a2d0a73df46e80b2e79fd430959d8f2b488)](https://circleci.com/gh/vuejs/vue-next)
 
-## Status: Pre-Alpha.
+## Status: Alpha.
 
-We have achieved most of the architectural goals and new features planned for v3:
+The current codebase has basic feature parity with v2.x, together with the changes proposed in [merged RFCs](https://github.com/vuejs/rfcs/pulls?q=is%3Apr+is%3Amerged+label%3A3.x). There is a simple webpack-based setup with Single-File Component support available [here](https://github.com/vuejs/vue-next-webpack-preview).
 
-- Compiler
-  - [x] Modular architecture
-  - [x] "Block tree" optimization
-  - [x] More aggressive static tree hoisting
-  - [x] Source map support
-  - [x] Built-in identifier prefixing (aka "stripWith")
-  - [x] Built-in pretty-printing
-  - [x] Lean ~10kb brotli-compressed browser build after dropping source map and identifier prefixing
-  - [x] SFC compiler (replacement of `@vue/component-compiler-utils`)
+At this stage, the only major work left is server-side rendering, which we are actively working on. In the meanwhile, we would like our users to start building small experimental apps using the alpha releases to help us identify bugs and stabilize the implementation.
 
-- Runtime
-  - [x] Significantly faster
-  - [x] Simultaneous Composition API + Options API support, **with typings**
-  - [x] Proxy-based change detection
-  - [x] Fragments
-  - [x] Portals
-  - [x] Suspense w/ `async setup()`
+Please note that there could still be undocumented behavior inconsistencies with 2.x. When you run into such a case, please make sure to first check if the behavior difference has already been proposed in an existing RFC. If the inconsistency is not part of an RFC, then it's likely unintended, and an issue should be opened (please make sure to use the [issue helper](https://new-issue.vuejs.org/?repo=vuejs/vue-next) when opening new issues).
 
-## Major TODOs:
+## Known Issues
 
-- [ ] Server-side rendering
+- There is currently no way to attach custom instance properties via `Vue.prototype`.
 
-Also note that the current implementation requires native ES2015+ in the runtime environment and does not support IE11 (yet).
+- The current implementation requires native ES2015+ in the runtime environment and does not support IE11 (yet).
 
 ## Contribution