Document function level vendor support

Author: dschep

README.md

@@ -192,9 +192,15 @@ dependencies to install:
 custom:
   pythonRequirements:
     vendor: ./vendored-libraries
+functions:
+  hello:
+    handler: hello.handler
+    vendor: ./hello-vendor # The option is also available at the function level
 ```
 
 
+
+
 ## Manual invocations
 
 The `.requirements` and `requirements.zip`(if using zip support) files are left