create repo, add some list comprehension examples

Author: rmccorm4

comprehension/list/README.md

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+# List Comprehension
+This directory is here to show the Speed Difference between using 
+List Comprehension vs. calling append() in a For Loop to create a list
+
+## Executing
+In a UNIX terminal, run:
+
+```
+time python list-append-loop.py
+```
+real	0m11.381s
+user	0m10.553s
+sys		0m0.824s
+
+```
+time python list-comp.py
+```
+real	0m4.228s
+user	0m3.428s
+sys		0m0.800s
+
+
+Here we see that the list comprehension was almost 3x as fast
+as the `list.append()` for loop. This is because everytime the
+append() method is called, it has to be looked up by the Interpreter,
+whereas the list comprehension can do it all at once.
+
+### Disclaimer
+Since this is a very simple example where we are just creating a list of
+numbers with the range() function, we can actually just cast the result
+of the range() function to a list and get even faster speed than the
+list comprehension. 
+
+To show a more practical example where you can't just cast it:
+
+```
+time python list-append-loop2.py
+```
+
+real	0m29.756s
+user	0m28.875s
+sys		0m0.880s
+
+
+```
+time python list-comp2.py
+```
+
+real	0m21.932s
+user	0m21.031s
+sys		0m0.900s
+
+In this practical example, we see that the list comprehension finished
+in about 2/3 of the time it took the for loop append() method, still
+significantly faster!
+
+All in all, it depends on the example, but list comprehensions are often
+faster than using a loop.

comprehension/list/cast-list.py

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+l = list(range(100000000))

comprehension/list/list-append-loop.py

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+l = []
+for i in range(100000000):
+	l.append(i)

comprehension/list/list-append-loop2.py

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+l = []
+for i in range(1000000):
+	l.append(i**2)

comprehension/list/list-comp.py

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+l = [x for x in range(100000000)]

comprehension/list/list-comp2.py

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+l = [x**2 for x in range(1000000)]