Stop configuring the root logger on import

[joeyleake]

Fixes #58

Problem

Importing meshcore_py called logging.basicConfig(level=logging.INFO) at
module load time, which configures Python's root logger for any
application that imports the library — silently overriding that
application's own logging setup. Separately, MeshCore.__init__ always
called logger.setLevel(...) even when neither debug nor only_error
was passed, forcing the level to INFO and overriding whatever the
embedding app had already configured.

Fix

• src/meshcore/__init__.py: removed the basicConfig() call. The module
  logger now just gets a NullHandler, so the library stays silent by
  default without emitting "no handlers found" warnings, and never touches
  the root logger.

• src/meshcore/meshcore.py: MeshCore.__init__ now only calls
  logger.setLevel(...) when the caller explicitly passes debug=True or
  only_error=True. The default case (neither flag set) leaves whatever
  level the embedding app configured alone, instead of forcing INFO.

• README.md: added a short note after the Debug Mode section showing the
  idiomatic pattern for consumers who want meshcore's logs:

  import logging
  logging.basicConfig(level=logging.INFO)
  logging.getLogger("meshcore").setLevel(logging.DEBUG)

Confirmed the logger name exported from __init__.py's __all__ isn't
referenced anywhere else in the package or examples, so removing
basicConfig doesn't change what that export points to for existing users
who import it directly.

Testing

Checked tests/ for anything depending on the implicit basicConfig (grep
for basicConfig/getLogger/caplog) — the only hits are
caplog.at_level(...) calls in test_reader.py, which set their own level
independently and needed no changes.

Full suite: 179 passed, 2 failed (test_send_cmd,
test_advert_path_preserves_embedded_zero_bytes) — both pre-existing on a
clean main checkout, unrelated to this change.