add nim books (#4078)

* add nim books

* edit title per comment

free-programming-books.md

@@ -2115,7 +2115,9 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### Nim
 
-* [Nim Basics](https://narimiran.github.io/nim-basics/)
+* [Nim Basics](https://narimiran.github.io/nim-basics) - narimiran
+* [Nim by Example](https://nim-by-example.github.io) - Flaviu Tamas
+* [Nim Days](https://xmonader.github.io/nimdays) - Ahmed Thabet
 
 
 ### NoSQL