Add comma for readability

I tripped over this sentence while reading it: "3) you will often need to customize your analyses and command-line tools often enable". I had to re-read it a few times before I realized that the "and" is the start of a new sentence. I am proposing a comma before the "and" because I think the comma increases readability.

After a bit of digging, I think I have figured out the source of my confusion. Sentence 3) is a compound sentence bound together by "and", and such sentences take a comma before the "and". Incidentally, the previous sentence is a compound sentence too. For reference, see rule /issues/3 on [here](https://webapps.towson.edu/ows/modulecomma.htm)

index.md

@@ -8,7 +8,7 @@ A lot of genomics analysis is done using command-line tools for three reasons:
 through a graphical user interface (GUI) allows you to automate repetitive tasks,    
 2) you will often need more compute power than is available on your personal computer, and 
 connecting to and interacting with remote computers requires a command-line interface, and    
-3) you will often need to customize your analyses and command-line tools often enable more 
+3) you will often need to customize your analyses, and command-line tools often enable more 
 customization than the corresponding GUI tools (if in fact a GUI tool even exists).   
 
 In a [previous lesson](http://www.datacarpentry.org/shell-genomics/), you learned how to use the bash shell to interact with your computer through a command line interface. In this