Update readme auth doc (#3169)

Mention how auth works in readme with link to nmp API.md and 
add realm login doc from node-minecraft-protocol's readme

docs/README.md

@@ -80,11 +80,11 @@ const mineflayer = require('mineflayer')
 
 const bot = mineflayer.createBot({
   host: 'localhost', // minecraft server ip
-  username: 'email@example.com', // minecraft username
+  username: 'Bot', // username or email, switch if you want to change accounts
   auth: 'microsoft' // for offline mode servers, you can set this to 'offline'
   // port: 25565,                // only set if you need a port that isn't 25565
   // version: false,             // only set if you need a specific version or snapshot (ie: "1.8.9" or "1.16.5"), otherwise it's set automatically
-  // password: '12345678'        // set if you want to use password-based auth (may be unreliable)
+  // password: '12345678'        // set if you want to use password-based auth (may be unreliable). If specified, the `username` must be an email
 })
 
 bot.on('chat', (username, message) => {
@@ -97,6 +97,26 @@ bot.on('kicked', console.log)
 bot.on('error', console.log)
 ```
 
+If `auth` is set to `microsoft`, you will be prompted to login to microsoft.com with a code in your browser. After signing in on your browser, 
+the bot will automatically obtain and cache authentication tokens in the local file system so you don't have to sign-in again. 
+To switch the account, update the supplied `username`. By default, cached tokens will be stored in your user's .minecraft folder.
+For more information on these options and others, see node-minecraft-protocol's [API doc](https://github.com/PrismarineJS/node-minecraft-protocol/blob/master/docs/API.md#mccreateclientoptions).
+
+#### Connecting to a Realm
+
+To join a Realm that your Minecraft account has been invited to, you can pass a `realms` object with a selector function like below.
+
+```js
+const client = mineflayer.createBot({
+  username: 'email@example.com', // minecraft username
+  realms: {
+    // This function is called with an array of Realms the account can join. It should return the one it wants to join.
+    pickRealm: (realms) => realms[0]
+  },
+  auth: 'microsoft'
+})
+```
+
 ### See what your bot is doing
 
 Thanks to the [prismarine-viewer](https://github.com/PrismarineJS/prismarine-viewer) project, it's possible to display in a browser window what your bot is doing.