Add description in the docs

docs/strategy-customization.md

@@ -532,6 +532,27 @@ If you want to use a strategy from a different directory you can pass `--strateg
 freqtrade trade --strategy AwesomeStrategy --strategy-path /some/directory
 ```
 
+### Derived strategies
+
+The strategies can be derived from other strategies. This avoids duplication of your custom strategy code. You can use this technique to override small parts of your main strategy, leaving the rest untouched:
+
+```
+class MyAwesomeStrategy(IStrategy):
+    ...
+    stoploss = 0.13
+    trailing_stop = False
+    # All other attributes and methods are here as they
+    # should be in any custom strategy...
+    ...
+
+class MyAwesomeStrategy2(MyAwesomeStrategy):
+    # Override something
+    stoploss = 0.08
+    trailing_stop = True
+```
+
+Both attributes and methods may be overriden, altering behavior of the original strategy in a way you need. The strategy classes may be located in the same module (python file with the source code of your strategy) or in different modules (different python files). In the latter case you need to properly import the strategy class you derive the new one from.
+
 ### Common mistakes when developing strategies
 
 Backtesting analyzes the whole time-range at once for performance reasons. Because of this, strategy authors need to make sure that strategies do not look-ahead into the future.