Document use of alternative header name parameter.

Also add language to previous code block for syntax highlighting on GH.

Author: matthewberryman

README.md

@@ -32,9 +32,15 @@ print(result)
 
 Authorization tokens can be added to the request using the client's `inject_token` method:
 
-```
+```py
 client.inject_token('very-long-and-secure-token')
 ```
+which defaults to http header name 'Authorization'. An alternative http header name for the token can be set by passing in the alternative header name, e.g. for 'x-api-key':
+
+```py
+client.inject_token('very-long-and-secure-token','x-api-key')
+```
+
 
 ## License