Add anonymous sign-ins environment variables to readme and example.env (

supabase#1515)

## What kind of change does this PR introduce?

Docs update

## What is the current behavior?

Currently, the README and example.env files don't contain the
environment variables to enable/disable anonymous sign-ins

## What is the new behavior?

Added "GOTRUE_EXTERNAL_ANONYMOUS_USERS_ENABLED" to README.md and
example.env

README.md

@@ -660,7 +660,6 @@ Default Content (if template is unavailable):
 <p><a href="{{ .ConfirmationURL }}">Change Email</a></p>
 ```
 
-
 ### Phone Auth
 
 `SMS_AUTOCONFIRM` - `bool`
@@ -717,6 +716,12 @@ Retrieve from hcaptcha or turnstile account
 
 Enforce reauthentication on password update.
 
+### Anonymous Sign-Ins
+
+`GOTRUE_EXTERNAL_ANONYMOUS_USERS_ENABLED` - `bool`
+
+Use this to enable/disable anonymous sign-ins.
+
 ## Endpoints
 
 Auth exposes the following endpoints:

example.env

@@ -168,6 +168,9 @@ GOTRUE_EXTERNAL_ZOOM_CLIENT_ID=""
 GOTRUE_EXTERNAL_ZOOM_SECRET=""
 GOTRUE_EXTERNAL_ZOOM_REDIRECT_URI="http://localhost:9999/callback"
 
+# Anonymous auth config
+GOTRUE_EXTERNAL_ANONYMOUS_USERS_ENABLED="false"
+
 # PKCE Config
 GOTRUE_EXTERNAL_FLOW_STATE_EXPIRY_DURATION="300s"