Link to redis:// scheme IANA registration in docs

Author: cvrebert

README.md

@@ -43,12 +43,14 @@ server or a different port, try:
 redis = Redis.new(:host => "10.0.1.1", :port => 6380, :db => 15)
 ```
 
-You can also specify connection options as an URL:
+You can also specify connection options as a [`redis://` URL][redis-url]:
 
 ```ruby
 redis = Redis.new(:url => "redis://:[email protected]:6380/15")
 ```
 
+[redis-url]: http://www.iana.org/assignments/uri-schemes/prov/redis
+
 By default, the client will try to read the `REDIS_URL` environment variable
 and use that as URL to connect to. The above statement is therefore equivalent
 to setting this environment variable and calling `Redis.new` without arguments.