Update ROADMAP.md

Author: cwmeijer

ROADMAP.md

@@ -2,7 +2,7 @@
 
 On the roadmap for the coming period are the following additions/changes.
 
-- Add Transformer architecture(s)
+- Add Transformer architecture(s) (See https://github.com/NLeSC/mcfly/issues/320)
 - literature review for architectures with respect to regression (and classification as well)
 - Don't load all models loaded in memory simultaneously
 - Add support for other metric classes (besides the string metrics from keras)