Move Hack-the-Kernel from advanced systems to core systems and change Three Easy Pieces from Required to Recommended

Author: waciumawanjohi

README.md

@@ -223,9 +223,10 @@ Courses | Duration | Effort | Prerequisites
 [Build a Modern Computer from First Principles: From Nand to Tetris](https://www.coursera.org/learn/build-a-computer) ([alt](http://www.nand2tetris.org/)) | 6 weeks | 7-13 hours/week | none
 [Build a Modern Computer from First Principles: Nand to Tetris Part II ](https://www.coursera.org/learn/nand2tetris2) | 6 weeks | 12-18 hours/week | From Nand to Tetris Part I
 [Introduction to Computer Networking](https://lagunita.stanford.edu/courses/Engineering/Networking-SP/SelfPaced/about)| 8 weeks | 4–12 hours/week | algebra, probability, basic CS
+[ops-class.org - Hack the Kernel](https://www.ops-class.org/) | 15 weeks | 6 hours/week | algorithms
 
 #### Readings
-- **Required** since Nand2Tetris does not go deep enough into operating systems: [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/)
+- **Recommended**: While Hack the Kernel recommends Modern Operating Systems as a textbook, we suggest using [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/).
 
 ### Core theory
 
@@ -359,7 +360,6 @@ Courses | Duration | Effort | Prerequisites
 [Computation Structures 1: Digital Circuits](https://www.edx.org/course/computation-structures-part-1-digital-mitx-6-004-1x-0) | 10 weeks | 6 hours/week | electricity, magnetism
 [Computation Structures 2: Computer Architecture](https://www.edx.org/course/computation-structures-2-computer-mitx-6-004-2x) | 10 weeks | 6 hours/week | Computation Structures 1
 [Computation Structures 3: Computer Organization](https://www.edx.org/course/computation-structures-3-computer-mitx-6-004-3x-0) | 10 weeks | 6 hours/week | Computation Structures 2
-[ops-class.org - Hack the Kernel](https://www.ops-class.org/) | 15 weeks | 6 hours/week | algorithms
 
 **<sup>1</sup> Note**:
 These courses assume knowledge of basic physics.