Add can_short to sample strategies

freqtrade/templates/base_strategy.py.j2

@@ -40,6 +40,9 @@ class {{ strategy }}(IStrategy):
     # Optimal timeframe for the strategy.
     timeframe = '5m'
 
+    # Can this strategy go short?
+    can_short: bool = False
+
     # Minimal ROI designed for the strategy.
     # This attribute will be overridden if the config file contains "minimal_roi".
     minimal_roi = {