feat: prepare "contrib" area

Signed-off-by: Jan Kowalleck <[email protected]>

Author: jkowalleck

cyclonedx/contrib/README.md

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+# CycloneDX Contrib Extensions
+
+This directory contains community-contributed functionality that extends the capabilities of the CycloneDX core library.  
+Unlike the modules in `../`, these features are not part of the official CycloneDX specification and may vary in stability, scope, or compatibility.
+
+## Contents
+- Utilities, helpers, and experimental features developed by the community
+- Optional add-ons that may facilitate or enhance use of the CycloneDX core library
+- Code that evolves independently of the CycloneDX specification
+
+## Notes
+- Contrib modules are optional and not required for strict compliance with the CycloneDX standard.
+- They may change more frequently than the core and are not guaranteed to follow the same versioning rules.
+- Users should evaluate these modules carefully and consult documentation or source comments for details.
+
+## Contributing
+Contributions are welcome. To add an extension:
+1. Follow the contribution guidelines in the main repository.
+2. Place your code in a clearly named subfolder or file under `contrib/`.
+3. Provide documentation and tests to ensure clarity and maintainability.  

cyclonedx/contrib/__init__.py

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+# This file is part of CycloneDX Python Library
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+# Copyright (c) OWASP Foundation. All Rights Reserved.
+
+
+"""
+Some features in this library are marked as contrib. These are community-provided extensions and are not part of the
+official standard. They are optional and may evolve independently from the core.
+"""
+
+__all__ = [
+    # there is no intention to export anything in here.
+    #
+]