Add an example of relational topic model. dongwookim-ml#4

Author: dongwookim-ml

README.md

@@ -12,7 +12,7 @@ Current implementations
 * Correlated topic Model
   * Variational inference
 * Relational topic model (VI)
-  * Exponential link function
+  * [Exponential link function](http://nbviewer.jupyter.org/github/arongdari/python-topic-model/blob/master/notebook/RelationalTopicModel_example.ipynb)
 * [Author-Topic model](http://nbviewer.jupyter.org/github/arongdari/python-topic-model/blob/master/notebook/AuthorTopicModel_example.ipynb)
 * [HMM-LDA](http://nbviewer.jupyter.org/github/arongdari/python-topic-model/blob/master/notebook/HMM_LDA_example.ipynb)
 * Discrete infinite logistic normal (DILN)