Add doco note on required extensions

Author: pramsey

examples/s3.sql

@@ -19,11 +19,11 @@
 -- https://cleverelephant-west-1.s3.amazonaws.com/META.json
 --
 -- SELECT * FROM s3_get(
---     'your_s3_access_key',
---     'your_s3_secret_key',
---     'us-west-1',
---     'cleverelephant-west-1',
---     'META.json'
+--     'your_s3_access_key',      -- access
+--     'your_s3_secret_key',      -- secret
+--     'us-west-1',               -- region
+--     'cleverelephant-west-1',   -- bucket
+--     'META.json'                -- object
 -- );
 --
 CREATE OR REPLACE FUNCTION s3_get(