@actions/github v3 using Octokit/core (#453)

* Rebuild to use @Octokit/Core

packages/github/README.md

@@ -4,7 +4,7 @@
 
 ## Usage
 
-Returns an authenticated Octokit client that follows the machine [proxy settings](https://help.github.com/en/actions/hosting-your-own-runners/using-a-proxy-server-with-self-hosted-runners). See https://octokit.github.io/rest.js for the API.
+Returns an authenticated Octokit client that follows the machine [proxy settings](https://help.github.com/en/actions/hosting-your-own-runners/using-a-proxy-server-with-self-hosted-runners) and correctly sets GHES base urls. See https://octokit.github.io/rest.js for the API.
 
 ```js
 const github = require('@actions/github');
@@ -17,7 +17,10 @@ async function run() {
     // https://help.github.com/en/actions/automating-your-workflow-with-github-actions/authenticating-with-the-github_token#about-the-github_token-secret
     const myToken = core.getInput('myToken');
 
-    const octokit = new github.GitHub(myToken);
+    const octokit = github.getOctokit(token)
+
+    // You can also pass in additional options as a second parameter to getOctokit
+    // const octokit = github.getOctokit(myToken, {userAgent: "MyActionVersion1"});
 
     const { data: pullRequest } = await octokit.pulls.get({
         owner: 'octokit',
@@ -34,8 +37,6 @@ async function run() {
 run();
 ```
 
-You can pass client options, as specified by [Octokit](https://octokit.github.io/rest.js/), as a second argument to the `GitHub` constructor.
-
 You can also make GraphQL requests. See https://github.com/octokit/graphql.js for the API.
 
 ```js
@@ -72,3 +73,25 @@ if (github.context.eventName === 'push') {
   core.info(`The head commit is: ${pushPayload.head}`)
 }
 ```
+
+## Extending the Octokit instance
+`@octokit/core` now supports the [plugin architecture](https://github.com/octokit/core.js#plugins). You can extend the GitHub instance using plugins. 
+
+For example, using the `@octokit/plugin-enterprise-server` you can now access enterprise admin apis on GHES instances.
+
+```ts
+import { GitHub, getOctokitOptions } from '@actions/github/lib/utils'
+import { enterpriseServer220Admin } from '@octokit/plugin-enterprise-server'
+
+const octokit = GitHub.plugin(enterpriseServer220Admin)
+// or override some of the default values as well 
+// const octokit = GitHub.plugin(enterpriseServer220Admin).defaults({userAgent: "MyNewUserAgent"})
+
+const myToken = core.getInput('myToken');
+const myOctokit = new octokit(getOctokitOptions(token))
+// Create a new user
+myOctokit.enterpriseAdmin.createUser({
+  login: "testuser",
+  email: "testuser@test.com",
+});
+```