Add instruction how to generate dokka documentation.

Author: MKoroteev-HORIS

docs/api_reference.md

@@ -1,7 +1,8 @@
 ### Browse
 
-https://htmlpreview.github.io/?https://raw.githubusercontent.com/JetBrains/lets-plot-kotlin/master/docs/api-reference/plot-api/index.html
+[API reference online.](https://htmlpreview.github.io/?https://raw.githubusercontent.com/JetBrains/lets-plot-kotlin/master/docs/api-reference/plot-api/index.html)
 
 ### How to generate
 
-TBD 
+1. Run Tasks > build > documentation > dokka.
+2. Set Reformat code and Update copyright flags during commit.