Update toolchain version, script, and README.md (swiftwasm#96)

The toolchain snapshot suggested in `README.md` is outdated. I think we should recommend using the script to install it automatically.

Resolves swiftwasm#95.

* Mention the `install-toolchain.sh` script in `README.md`

* Update toolchain version, script, and `README.md`

* Update example code in `README.md`

* Update .swift-version

* Update Swift version in README.md

Author: MaxDesiatov

.swift-version

@@ -1 +1 @@
-wasm-5.3-SNAPSHOT-2020-10-02-a
+wasm-5.3-SNAPSHOT-2020-10-20-a

README.md

@@ -4,23 +4,7 @@
 
 Swift framework to interact with JavaScript through WebAssembly.
 
-## Requirements
-
-This library only supports [`swiftwasm/swift`](https://github.com/swiftwasm/swift) distribution toolchain. Please install Swift for WebAssembly toolchain from [Release Page](https://github.com/swiftwasm/swift/releases)
-
-The toolchains can be installed via [`swiftenv`](https://github.com/kylef/swiftenv) like official nightly toolchain.
-
-e.g.
-
-```sh
-
-$ swiftenv install https://github.com/swiftwasm/swift/releases/download/swift-wasm-5.3-SNAPSHOT-2020-08-10-a/swift-wasm-5.3-SNAPSHOT-2020-08-10-a-osx.tar.gz
-$ swift --version
-Swift version 5.3-dev (LLVM 09686f232a, Swift 5a196c7f13)
-Target: x86_64-apple-darwin19.6.0
-```
-
-## Usage
+## Getting started
 
 This JavaScript code
 
@@ -48,13 +32,11 @@ Can be written in Swift using JavaScriptKit
 ```swift
 import JavaScriptKit
 
-let alert = JSObject.global.alert.function!
-let document = JSObject.global.document.object!
+let document = JSObject.global.document
 
-let divElement = document.createElement!("div").object!
+let divElement = document.createElement("div")
 divElement.innerText = "Hello, world"
-let body = document.body.object!
-_ = body.appendChild!(divElement)
+_ = document.body.appendChild(divElement)
 
 struct Owner: Codable {
   let name: String
@@ -68,7 +50,82 @@ struct Pet: Codable {
 let jsPet = JSObject.global.pet
 let swiftPet: Pet = try JSValueDecoder().decode(from: jsPet)
 
-alert("Swift is running on browser!")
+JSObject.global.alert("Swift is running in the browser!")
+```
+
+### Usage in a browser application
+
+The easiest way to get started with JavaScriptKit in your browser app is with [the `carton`
+bundler](https://carton.dev).
+
+As a part of these steps
+you'll install `carton` via [Homebrew](https://brew.sh/) on macOS (unfortunately you'll have to build
+it manually on Linux). Assuming you already have Homebrew installed, you can create a new app
+that uses JavaScriptKit by following these steps:
+
+1. Install `carton`:
+
+```
+brew install swiftwasm/tap/carton
+```
+
+If you had `carton` installed before this, make sure you have version 0.6.1 or greater:
+
+```
+carton --version
 ```
 
-Please see [Example](https://github.com/swiftwasm/JavaScriptKit/tree/main/Example) directory for more information
+2. Create a directory for your project and make it current:
+
+```
+mkdir SwiftWasmApp && cd SwiftWasmApp
+```
+
+3. Initialize the project from a template with `carton`:
+
+```
+carton init --template basic
+```
+
+4. Build the project and start the development server, `carton dev` can be kept running
+   during development:
+
+```
+carton dev
+```
+
+5. Open [http://127.0.0.1:8080/](http://127.0.0.1:8080/) in your browser and a developer console
+   within it. You'll see `Hello, world!` output in the console. You can edit the app source code in
+   your favorite editor and save it, `carton` will immediately rebuild the app and reload all
+   browser tabs that have the app open.
+
+You can also build your project with webpack.js and a manually installed SwiftWasm toolchain. Please
+see the following sections and the [Example](https://github.com/swiftwasm/JavaScriptKit/tree/main/Example)
+directory for more information in this more advanced use case.
+
+### Manual toolchain installation
+
+This library only supports [`swiftwasm/swift`](https://github.com/swiftwasm/swift) distribution
+toolchain. The toolchain can be installed via [`swiftenv`](https://github.com/kylef/swiftenv), in
+the same way as the official Swift nightly toolchain.
+
+You have to install the toolchain manually when working on the source code JavaScriptKit itself,
+especially if you change anything in the JavaScript runtime parts. This is because the runtime is
+embedded in `carton` and currently can't be replaced dynamically with the JavaScript code you've
+updated locally.
+
+Just pass a toolchain archive URL for [the latest SwiftWasm 5.3
+snapshot](https://github.com/swiftwasm/swift/releases) appropriate for your platform:
+
+```sh
+$ swiftenv install https://github.com/swiftwasm/swift/releases/download/swift-wasm-5.3-SNAPSHOT-2020-10-20-a/swift-wasm-5.3-SNAPSHOT-2020-10-20-a-macos_x86_64.pkg
+```
+
+You can also use the `install-toolchain.sh` helper script that uses a hardcoded toolchain snapshot:
+
+```sh
+$ ./scripts/install-toolchain.sh
+$ swift --version
+Swift version 5.3 (swiftlang-5.3.0)
+Target: x86_64-apple-darwin19.6.0
+```

scripts/install-toolchain.sh

@@ -18,13 +18,13 @@ fi
 
 case $(uname -s) in
   Darwin)
-    toolchain_download="$swift_tag-osx.tar.gz"
+    toolchain_download="$swift_tag-macos_x86_64.pkg"
   ;;
   Linux)
     if [ $(grep RELEASE /etc/lsb-release) == "DISTRIB_RELEASE=18.04" ]; then
-      toolchain_download="$swift_tag-ubuntu18.04.tar.gz"
+      toolchain_download="$swift_tag-ubuntu18.04_x86_64.tar.gz"
     elif [ $(grep RELEASE /etc/lsb-release) == "DISTRIB_RELEASE=20.04" ]; then
-      toolchain_download="$swift_tag-ubuntu20.04.tar.gz"
+      toolchain_download="$swift_tag-ubuntu20.04_x86_64.tar.gz"
     else
       echo "Unknown Ubuntu version"
       exit 1