[docs] add simple declaration of data format and demo

Author: tswsxk

XCD/IRT/MXIRT/README.md

@@ -0,0 +1,16 @@
+# MXIRT
+
+Implementation of IRT in `mxnet`.
+
+## Data Format
+The input to our model should be a triplet csv, for example:
+```
+user_id,item_id,score
+1615,12977,1
+782,13124,0
+1084,16475,0
+593,8690,0
+```
+
+## Demo
+Demo could be found in [here](../../../scripts/reproduction/xcd/mxirt.ipynb)