added Learn Functional Programming (#11778)

Author: goranjovic

books/free-programming-books-subjects.md

@@ -811,6 +811,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 * [Flow based Programming](https://jpaulmorrison.com/fbp/) - J Paul Morrison
 * [Introduction to Functional Programming](https://www.cl.cam.ac.uk/teaching/Lectures/funprog-jrh-1996/) - J. Harrison
+* [Learn Functional Programming](https://learn-functional-programming.com) - G. Jovic (HTML)
 * [Making Sense of Stream Processing](https://assets.confluent.io/m/2a60fabedb2dfbb1/original/20190307-EB-Making_Sense_of_Stream_Processing_Confluent.pdf) - Martin Kleppmann (PDF)
 * [Mostly Adequate Guide to Functional Programming](https://mostly-adequate.gitbooks.io/mostly-adequate-guide/content/) - Mostly Adequate Core Team
 * [The Pure Function Pipeline Data Flow v3.0 ---- the Grand Unified Programming Theory](https://github.com/linpengcheng/PurefunctionPipelineDataflow) - Lin Pengcheng