chore: adds UMD instructions to README

Author: awalias

README.md

@@ -17,6 +17,24 @@ import { createClient } from '@supabase/supabase-js'
 const supabase = createClient('https://xyzcompany.supabase.co', 'public-anon-key')
 ```
 
+### UMD
+
+You can now use plain `<script>`s to import supabase-js from CDNs, like 
+
+`<script src="https://cdn.jsdelivr.net/npm/@supabase/supabase-js"></script>`
+or
+`<script src="https://unpkg.com/@supabase/supabase-js"></script>`.
+
+Then you can use it from a global `supabase` variable:
+
+```html
+<script>
+  const { createClient } = supabase
+  supabase = createClient('https://xyzcompany.supabase.co', 'public-anon-key')
+...
+</script>
+```
+
 ## Sponsors
 
 We are building the features of Firebase using enterprise-grade, open source products. We support existing communities wherever possible, and if the products don’t exist we build them and open source them ourselves. Thanks to these sponsors who are making the OSS ecosystem better for everyone.