Merge pull request #1048 from spamegg1/master

alternate links for Computation Structures

README.md

@@ -363,11 +363,11 @@ Courses | Duration | Effort | Prerequisites
 `system call interface`
 `and more`
 
-Courses | Duration | Effort | Prerequisites
-:-- | :--: | :--: | :--:
-[Computation Structures 1: Digital Circuits](https://learning.edx.org/course/course-v1:MITx+6.004.1x_3+3T2016) | 10 weeks | 6 hours/week | [Nand2Tetris II](https://www.coursera.org/learn/nand2tetris2)
-[Computation Structures 2: Computer Architecture](https://learning.edx.org/course/course-v1:MITx+6.004.2x+3T2015) | 10 weeks | 6 hours/week | Computation Structures 1
-[Computation Structures 3: Computer Organization](https://learning.edx.org/course/course-v1:MITx+6.004.3x_2+1T2017) | 10 weeks | 6 hours/week | Computation Structures 2
+Courses | Duration | Effort | Prerequisites | Notes
+:-- | :--: | :--: | :--: | :--:
+[Computation Structures 1: Digital Circuits](https://learning.edx.org/course/course-v1:MITx+6.004.1x_3+3T2016) [alt1](https://ocw.mit.edu/courses/6-004-computation-structures-spring-2017/) [alt2 ](https://ocw.mit.edu/courses/6-004-computation-structures-spring-2009/) | 10 weeks | 6 hours/week | [Nand2Tetris II](https://www.coursera.org/learn/nand2tetris2) | Alternate links contain all 3 courses.
+[Computation Structures 2: Computer Architecture](https://learning.edx.org/course/course-v1:MITx+6.004.2x+3T2015) | 10 weeks | 6 hours/week | Computation Structures 1 |
+[Computation Structures 3: Computer Organization](https://learning.edx.org/course/course-v1:MITx+6.004.3x_2+1T2017) | 10 weeks | 6 hours/week | Computation Structures 2 |
 
 ### Advanced theory