Move optional Intro CS courses to extras

Since we want every course in Intro CS and Core CS to be required, I have removed
the optional courses from Intro CS. But they are still good courses so I made
a new section in extras/courses.md to house courses on online learning.

I also added a new course under this new section, the sequel to Learning How
to Learn.

README.md

@@ -109,9 +109,7 @@ and is knowledgeable enough now to decide which electives to take.
 ## Intro CS
 
 Use the first course, CS50, to determine if Computer Science is right for you.
-Only proceed in the curriculum if it really excites you.
-Feel free to use the second and third **optional** courses to gain the fundamental skills you need to excel at teaching yourself Computer Science;
-they are very lightweight courses so it's suggested take them concurrently with another course.
+Only proceed in the curriculum if it really excites you!
 
 **Topics covered**:
 imperative programming;
@@ -127,8 +125,6 @@ and more.
 Courses | Duration | Effort | Prerequisites
 :-- | :--: | :--: | :--:
 [Introduction to Computer Science - CS50](https://www.edx.org/course/introduction-computer-science-harvardx-cs50x#!) ([alt](https://cs50.harvard.edu/)) | 12 weeks | 10-20 hours/week | none
-[Learning How to Learn](https://www.coursera.org/learn/learning-how-to-learn) | 4 weeks | 2 hours/week | none
-[Effective Thinking Through Mathematics](https://www.edx.org/course/effective-thinking-through-mathematics-utaustinx-ut-9-01x-0) | 4 weeks | 2 hours/week | none
 
 ## Core CS
 

extras/courses.md

@@ -23,6 +23,7 @@ Courses | Duration | Effort
 
 Courses | Duration | Effort
 :-- | :--: | :--:
+[Effective Thinking Through Mathematics](https://www.edx.org/course/effective-thinking-through-mathematics-utaustinx-ut-9-01x-0) | 4 weeks | 2 hours/week | none
 [Introduction to Probability and Data](https://www.coursera.org/learn/probability-intro)| - | -
 [Linear Algebra (Strang)](https://ocw.mit.edu/courses/mathematics/18-06-linear-algebra-spring-2010/index.htm) | - | -
 
@@ -62,4 +63,11 @@ Courses | Duration | Effort
 [Intro to Machine Learning](https://www.udacity.com/course/intro-to-machine-learning--ud120)| 10 weeks | 6-10 hours/week
 [Machine Learning for Data Science and Analytics](https://www.edx.org/course/machine-learning-data-science-analytics-columbiax-ds102x-0)| 5 weeks | 7-10 hours/week
 [Processing Big Data with Azure HDInsight](https://www.edx.org/course/processing-big-data-azure-hdinsight-microsoft-dat202-1x-0)| 5 weeks | 3-4 hours/week
-[Big Data Science with the BD2K-LINCS Data Coordination and Integration Center](https://www.coursera.org/course/bd2klincs)| 7 weeks | 4-5 hours/week
+[Big Data Science with the BD2K-LINCS Data Coordination and Integration Center](https://www.coursera.org/course/bd2klincs)| 7 weeks | 4-5 hours/week
+
+# Online Learning - Great Courses
+
+Courses | Duration | Effort
+:-- | :--: | :--:
+[Learning How to Learn](https://www.coursera.org/learn/learning-how-to-learn) | 4 weeks | 2 hours/week
+[Mindshift](https://www.coursera.org/learn/mindshift) | 4 weeks | 2 hours/week