Added to README for supporting Docker v19.03

This commit adds an example for `use_partial_metadata` for Docker v19.03 logs.

Also fixes the previous example for handling Docker logs (changed the
key from 'message' to 'log' as Docker's fluentd driver wraps the
container's log line in the 'log' key as seen in this commit
moby/moby@361a582#diff-4e76640044eca60a4a45cd8631049282R112)

Author: raytung

README.md

@@ -168,13 +168,24 @@ Handle Docker logs splitted in several parts (using `partial_message`), and do n
 ```aconf
 <filter>
   @type concat
-  key message
+  key log
   partial_key partial_message
   partial_value true
   separator ""
 </filter>
 ```
 
+(Docker v19.03+) Handle Docker logs splitted in several parts (using `use_partial_metadata`), and do not add new line between parts.
+
+```aconf
+<filter>
+  @type concat
+  key log
+  use_partial_metadata true
+  separator ""
+</filter>
+```
+
 Handle Docker logs splitted in several parts (using newline detection), and do not add new line between parts (prior to Docker 18.06).
 
 ```aconf