Add NLP example to readme (#114)

* Added simple usage for NLP package to readme

* Added example grouping sentiment by author

---------

Co-authored-by: Jonas Weich <jns.wch@gmail.com>

README.md

@@ -103,8 +103,28 @@ ax[1] = vis.radar(df, ax=ax[1], color='C1', alpha=0)
   <img src="examples/radar.svg">
 </p>
 
+## 5. Natural Language Processing
 
-## 5. Command Line Interface
+### 5.1 Add Sentiment 
+
+```python
+from chatminer.nlp import add_sentiment
+
+df_sentiment = add_sentiment(df)
+```
+### 5.2 Example Plot: Sentiment per Author in Groupchat
+
+```python
+df_grouped = df_sentiment.groupby(['author', 'sentiment']).size().unstack(fill_value=0)
+ax = df_grouped.plot(kind='bar', stacked=True, figsize=(8, 3))
+```
+
+<p align="center">
+  <img src="examples/nlp.svg">
+</p>
+
+
+## 6. Command Line Interface
 The CLI supports parsing chat logs into csv files.
 As of now, you **can't** create visualizations from the CLI directly.