Added course for Data Structures (#5828)

Author: anirudhgray

courses/free-courses-en.md

@@ -94,6 +94,7 @@
 * [Algorithms](https://www.youtube.com/playlist?list=PLDN4rrl48XKpZkf03iYFl-O29szjTrs_O) - Abdul Bari
 * [Berkeley University CS 61B: Data Structures](http://datastructur.es/sp16/)
 * [Data Structures](https://www.youtube.com/playlist?list=PL2_aWCzGMAwI3W_JlcBbtYTwiQSsOTa6P) - mycodeschool
+* [Data Structures](https://stepik.org/course/579/syllabus) - Niema Moshiri and Liz Izhikevich (Stepik)
 * [Data Structures and Algorithms Specialization](https://www.coursera.org/specializations/data-structures-algorithms) - UC San Diego, HSE University
 * [Data Structures Easy to Advanced Course - Full Tutorial from a Google Engineer](https://www.youtube.com/watch?v=RBSGKlAvoiM) - William Fiset - (freeCodeCamp)
 * [IIT Bombay Foundation of Data Structures (CS213.1x)](https://courses.edx.org/courses/course-v1:IITBombayX+CS213.1x+1T2017/course/)