Add docs for constant folding

Author: dflook

docs/source/transforms/constant_folding.py

@@ -0,0 +1,2 @@
+SECONDS_IN_A_DAY = 60 * 60 * 24
+SECONDS_IN_A_WEEK = SECONDS_IN_A_DAY * 7

docs/source/transforms/constant_folding.rst

@@ -0,0 +1,25 @@
+Constant Folding
+================
+
+This transform evaluates constant expressions with literal operands when minifying and replaces the expression with the resulting value, if the value is shorter than the expression.
+
+There are some limitations, notably the division and power operators are not evaluated.
+
+This will be most effective with numeric literals.
+
+This transform is always safe and enabled by default. Disable by passing the ``constant_folding=False`` argument to the :func:`python_minifier.minify` function,
+or passing ``--no-constant-folding`` to the pyminify command.
+
+Example
+-------
+
+Input
+~~~~~
+
+.. literalinclude:: constant_folding.py
+
+Output
+~~~~~~
+
+.. literalinclude:: constant_folding.min.py
+    :language: python

docs/source/transforms/index.rst

@@ -19,6 +19,7 @@ They can be enabled or disabled through the minify function, or passing options
    convert_posargs_to_args
    preserve_shebang
    remove_explicit_return_none
+   constant_folding
 
 .. toctree::
    :caption: Disabled by default