doc: add leaf list documentation

binary_tree/leaf_list/README.md

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+# leaf list
+
+Write a function, leaf_list, that takes in the root of a binary tree and returns a list containing the values of all leaf nodes in left-to-right order.
+
+## test_00:
+
+```python
+a = Node("a")
+b = Node("b")
+c = Node("c")
+d = Node("d")
+e = Node("e")
+f = Node("f")
+
+a.left = b
+a.right = c
+b.left = d
+b.right = e
+c.right = f
+
+#      a
+#    /   \
+#   b     c
+#  / \     \
+# d   e     f
+
+leaf_list(a) # -> [ 'd', 'e', 'f' ] 
+```
+
+## test_01:
+
+```python
+a = Node("a")
+b = Node("b")
+c = Node("c")
+d = Node("d")
+e = Node("e")
+f = Node("f")
+g = Node("g")
+h = Node("h")
+
+a.left = b
+a.right = c
+b.left = d
+b.right = e
+c.right = f
+e.left = g
+f.right = h
+
+#      a
+#    /   \
+#   b     c
+#  / \     \
+# d   e     f
+#    /       \
+#   g         h
+
+leaf_list(a) # -> [ 'd', 'g', 'h' ]
+```
+
+## test_02:
+
+```python
+a = Node(5)
+b = Node(11)
+c = Node(54)
+d = Node(20)
+e = Node(15)
+f = Node(1)
+g = Node(3)
+
+a.left = b
+a.right = c
+b.left = d
+b.right = e
+e.left = f
+e.right = g
+
+#        5
+#     /    \
+#    11    54
+#  /   \
+# 20   15
+#      / \
+#     1  3
+
+leaf_list(a) # -> [ 20, 1, 3, 54 ]
+```
+
+## test_03:
+
+```python
+x = Node('x')
+
+#      x
+
+leaf_list(x) # -> [ 'x' ]
+```
+
+## test_04:
+
+```python
+leaf_list(None) # -> [ ]
+```