Docs: need usage example in README.md

1. using Pike in interactive Python
2. using Pike like an SMB client
3. using Pike to write unittests
4. using Pike with pytest

The existing tests under pike.test aren't necessarily the best examples and we don't really want people copying the bad patterns.

Would be good to have some python3, pytest, `pike.test.TreeConnect` examples in place showing the most common API calls and linking into the generated documentation for full details.