Update README to v3 behavior (#789)

Move all of it to the documentation so that we don't need to maintain it
in multiple locations.

[skip ci]

Closes #755

README.md

@@ -10,44 +10,7 @@ The core AppSignal for Node.js library.
 
 ## Installation
 
-Please follow our [installation guide](https://docs.appsignal.com/guides/new-application.html) in our documentation.
-
-First, [sign up][appsignal-sign-up] for an AppSignal account and run our automated install tool, which will install `@appsignal/nodejs` and any relevant integrations to your project:
-
-```bash
-npx @appsignal/cli install
-```
-
-You can also skip the automated tool and add `@appsignal/nodejs` to your `package.json` on the command line with `npm`/`yarn`:
-
-```bash
-yarn add @appsignal/nodejs
-npm install --save @appsignal/nodejs
-```
-
-Alternatively, you can manually add the `@appsignal/nodejs` package to your `package.json`. Then, run `yarn install`/`npm install`.
-
-> Installing the AppSignal for Node.js integration builds a native extension. In order to compile it, macOS users will need to install the [Xcode Developer Tools](https://osxdaily.com/2014/02/12/install-command-line-tools-mac-os-x/). Linux users will need the dependencies outlined here. Windows is not supported.
-
-You can then import and use the package in your bundle:
-
-```js
-const { Appsignal } = require("@appsignal/nodejs");
-
-const appsignal = new Appsignal({
-  active: true,
-  name: "<YOUR APPLICATION NAME>",
-  pushApiKey: "<YOUR API KEY>"
-});
-
-// ...all the rest of your code goes here!
-```
-
-> In order to auto-instrument modules, the Appsignal module must be both **required** and **initialized** before any other package.
-
-## Extension
-
-This package also contains the C++ extension, and the install script for the agent. The native extension is a bridge between the Node.js runtime and [our agent](https://docs.appsignal.com/appsignal/how-appsignal-operates.html#agent).
+Please follow our [installation guide](https://docs.appsignal.com/nodejs/3.x/installation) in our documentation. We try to automatically instrument as many packages as possible, but may not always be able to. Make to sure follow any [instructions to add manual instrumentation](https://docs.appsignal.com/nodejs/3.x/integrations).
 
 ## Development