Update Additional-Reading.md (#6496)
Update Additional-Reading.md (#6496)

* Update Additional-Reading.md

#6132
- 'What is Reactive Programming?' was wrong link, so deleted.
- First line was edited.

* Update Additional-Reading.md

- add 'What is Reactive Programming?' link.

artem-zinnatullin/RxJava,artem-zinnatullin/RxJava