update README on use of encode/decode/transocde in vcl_init()/vcl_fini()

README.rst

@@ -404,6 +404,12 @@ string is an illegal format for the decoding scheme.
 If the ``blob`` object constructor fails, then the VCL program will
 fail to load, and the VCC compiler will emit an error message.
 
+For varnish versions before 5.0, the VMOD functions -- ``encode()``,
+``decode()`` and ``transcode()`` -- may not be called in
+``vcl_init()`` or ``vcl_fini()`` (since a workspace must be
+available). Use in ``vcl_init()`` will also cause the VCL program to
+fail with a VCC compiler error.
+
 If any of the VMOD's methods or functions fail at runtime, then an
 error message will be written to the Varnish log using the tag
 ``VCL_Error``. The encoders and decoders return ``NULL`` on failure;