Fix broken links

I also added a note about the "Testing vs checking" article since so much has happened with it over the years since we first included it.

Author: NicMcPhee

README.md

@@ -31,7 +31,7 @@ would encourage everyone in the course to do the same.
   * Nice review of why C has header files, and how they're typically used.
 * <http://en.wikipedia.org/wiki/Boolean_datatype#C>
   * On the craziness that is booleans (or the lack thereof) in C.
-* <http://en.wikipedia.org/wiki/Struct_(C_programming_language>)
+* <http://en.wikipedia.org/wiki/Struct_(C_programming_language)>
   * A nice overview of C structs
 
 <http://www.cprogramming.com/tutorial.html#ctutorial> has a decent
@@ -49,7 +49,7 @@ particularly useful lessons in this context would be:
 
 ### Checking vs. Exploration
 
-[As this article points out nicely](http://www.developsense.com/2009/08/testing-vs-checking.html),
+[As this article points out nicely](https://www.developsense.com/blog/2009/08/testing-vs-checking/),
 it's useful to make distinction between checking (which is what we
 typically call testing in our courses) and exploration (he calls it
 testing, but I prefer exploration given that "testing" means other
@@ -62,11 +62,19 @@ a result one often does the exploring off to the side, with the
 intention of throwing it away. I bring all this up because I suspect
 there will be a fair amount of exploring that goes on during this
 pre-lab and the following labs.
+
 Try to be intentional and honest about that. Step off to the side and
 try a little exploratory code to figure out if you've got an idea worked
 out correctly. Then throw away that "quick and dirty" code, and bring
 your new knowledge back to the project at hand.
 
+(It's worth noting that
+[the article linked above](https://www.developsense.com/blog/2009/08/testing-vs-checking/)
+has clearly had a long and complex history, as indicated by the long note at the top with
+links to subsequent pieces he wrote in response to (mis) interpretations of his ideas.
+All that said, I still think there's a lot of really good stuff here, and I think his
+distinction between testing and exploration is entirely valid.)
+
 ### Compiling and running a C program
 
 In the exercise below you'll need to edit, (re)compile, and run the C