Update readme.md

Author: doruirimescu

Trading/utils/criterion/readme.md

@@ -1,4 +1,49 @@
 # Numerical and logical expression wrapper
+
+Wrapper for logical expressions that can be debugged, printed, enabled and disabled.
+
+Here is a practical example:
+```python
+from Trading.utils.criterion.expression import Numerical, and_
+import operator
+
+payout_criterion = Numerical(
+    "Payout ratio", operator.lt, payout_ratio, max_payout_ratio
+)
+yield_criterion = Numerical(
+    "Dividend yield", operator.ge, dividend_yield, min_yield
+)
+debt_criterion = Numerical(
+    "Debt-to-equity ratio", operator.le, debt_equity, max_debt_equity
+)
+roe_criterion = Numerical("Return on equity", operator.ge, roe, min_roe)
+free_cashflow_criterion = Numerical(
+    "Free cash flow", operator.ge, free_cashflow, min_free_cashflow
+)
+criteria = and_(
+    payout_criterion,
+    yield_criterion,
+    debt_criterion,
+    roe_criterion,
+    free_cashflow_criterion,
+)
+print(criteria.formatted())
+```
+
+*Console output:*
+```txt
+( (Payout ratio 250.0 < 80 False) & 
+( (Dividend yield 12.93 >= 2.0 True) & 
+( (Debt-to-equity ratio 1.16 <= 0.5 False) & 
+( (Return on equity 4.45 >= 10.0 False) & 
+(Free cash flow 200260256 >= 10000000.0 True) False) False) False) False)
+```
+Whenever you have complex criteria in your program, you should really use this framework. 
+
+Calling `.debug` on a criterion will list the conditions that make it evaluate to `False`. 
+
+By inspecting this output, you can painlessly debug why your criteria doesn't evaluate to `True` when expected.
+
 ```python
 import operator
 roe = Threshold("Return on Equity: ", operator.ge, 10.0)