Create incdec.md

docs/features/incdec.md

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+---
+icon: dot
+title: Increment Statement
+---
+
+pkpy provides `++i` and `--j` statements to operate a simple named `int` variable.
+
++ `++i` is equivalent to `i+=1`, but much faster
++ `--j` is equivalent to `j-=1`, but much faster
+
+## Example
+
+```python
+a = 1
+++a
+assert a == 2
+
+def f(a):
+  --a
+  return a
+
+assert f(3) == 2
+```