Create str-join.md

Create a tag to showcase `str.join`

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+**Joining Iterables**
+
+Suppose you want to nicely display a list (or some other iterable). The naive solution would be something like this.
+```py
+colors = ['red', 'green', 'blue', 'yellow']
+output = ""
+separator = ", "
+for color in colors:
+    output += color + separator
+print(output) # Prints 'red, green, blue, yellow, '
+```
+However, this solution is flawed. The separator is still added to the last color, and it is slow.
+
+The better way is to use `str.join`.
+```py
+colors = ['red', 'green', 'blue', 'yellow']
+separator = ", "
+print(separator.join(colors)) # Prints 'red, green, blue, yellow'
+```
+This method is much simpler, faster, and solves the problem of the extra separator. An important thing to note is that you can only `str.join` strings. For a list of ints, 
+you must convert each element to a string before joining.
+```py
+integers = [1, 3, 6, 10, 15]
+print(", ".join(str(e) for e in integers)) # Prints '1, 3, 6, 10, 15'
+```