Fix #207. Update README: headers for dictionaries.

Author: ilya112358

README.md

@@ -121,10 +121,22 @@ dictionaries or named tuples:
 ```pycon
 >>> print(tabulate({"Name": ["Alice", "Bob"],
 ...                 "Age": [24, 19]}, headers="keys"))
-  Age  Name
------  ------
-   24  Alice
-   19  Bob
+Name      Age
+------  -----
+Alice      24
+Bob        19
+```
+
+When data is a list of dictionaries, a dictionary can be passed as `headers`
+to replace the keys with other column labels:
+
+```pycon
+>>> print(tabulate([{1: "Alice", 2: 24}, {1: "Bob", 2: 19}], 
+...                headers={1: "Name", 2: "Age"}))
+Name      Age
+------  -----
+Alice      24
+Bob        19
 ```
 
 ### Row Indices