add angstromctf.com (#3582)

* -_-

* add refactoring

* add refactoring

Co-authored-by: rndd <you@example.com>

problem-sets-competitive-programming.md

@@ -39,6 +39,7 @@
 
 ### Information security
 
+* [ångstromCTF](https://angstromctf.com)
 * [CTFtime](https://ctftime.org)