guide: add .dvcignore example when we depend on dir with Python code

[shcheklein]

And exclude .pyc.

[jorgeorpinel]

Can you expand a little on "when we depend on dir with Python code" @shcheklein ? 🙂

[shcheklein]

Sure, @jorgeorpinel . In case like dvc run -d src/ ..., src might change not because of the actual changes to some code file in the src/ but because Python might generate __pycache__ subdirectory, or some .pyc file. We would like to ignore them when we calculate md5 for the directory.