Document the VMOD blobcode requirement for make check.

INSTALL.rst

@@ -27,6 +27,9 @@ the ``configure`` option
 ``--with-libgcrypt-prefix=$LIBGCRYPT_PREFIX``, where
 ``$LIBGCRYPT_PREFIX`` is the alternative path.
 
+The ``make check`` step requires that the VMOD blobcode is installed
+for the Varnish instance against which the build is executed.
+
 If you have installed Varnish in a non-standard directory, call
 ``autogen.sh`` and ``configure`` with the ``PKG_CONFIG_PATH``
 environment variable pointing to the appropriate path. For example,