Update contributing guides

Author: gyermolenko

README.md

@@ -79,22 +79,31 @@ Contributing
 When an implementation is added or modified, please review the following guidelines:
 
 ##### Output
-All files with example patterns have `### OUTPUT ###` section at the bottom.
+All files with example patterns have `### OUTPUT ###` section at the bottom 
+(migration to OUTPUT = """...""" is in progress).
 
 Run `append_output.sh` (e.g. `./append_output.sh borg.py`) to generate/update it.
 
 ##### Docstrings
 Add module level description in form of a docstring with links to corresponding references or other useful information.
 
-[strategy.py](patterns/behavioral/strategy.py) has a good example of detailed description,
+Add "Examples in Python ecosystem" section if you know some. It shows how patterns could be applied to real-world problems.
+
+[facade.py](patterns/structural/facade.py) has a good example of detailed description,
 but sometimes the shorter one as in [template.py](patterns/behavioral/template.py) would suffice.
 
 In some cases class-level docstring with doctest would also help (see [adapter.py](patterns/structural/adapter.py))
+but readable OUTPUT section is much better.
+
 
 ##### Python2/3 compatibility
 Try to keep it (discussion is held in [issue #208](https://github.com/faif/python-patterns/issues/208))
 - use new style classes (inherit from `object`)
-- use `from future import print`
+- use `from __future__ import print_function`
 
 ##### Update README
 When everything else is done - update corresponding part of README.
+
+
+##### Travis CI
+Please run `flake8` and `pytest` commands locally to be sure your changes will pass CI .

patterns/behavioral/visitor.py

@@ -13,7 +13,9 @@
 - "8.22. Implementing the Visitor Pattern Without Recursion"
 
 *Examples in Python ecosystem:
-Python's ast.NodeVisitor: https://github.com/python/cpython/blob/master/Lib/ast.py#L250
+- Python's ast.NodeVisitor: https://github.com/python/cpython/blob/master/Lib/ast.py#L250
+which is then being used e.g. in tools like `pyflakes`.
+- `Black` formatter tool implements it's own: https://github.com/ambv/black/blob/master/black.py#L718
 """