Add README.md for strings module

Author: sebastienrousseauhsbc

encryption_helper/common/README.md

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+<p align="center">
+  <img src="https://www.hsbc.com/-/files/hsbc/header/hsbc-logo-200x25.svg" alt="HSBC Logo" width="200" title="HSBC Logo">
+</p>
+
+<h1 align="center">Encryption Helper Python - Strings Module</h1>
+
+<p align="center">
+  <img src="../../assets/banner.jpg" alt="Encryption Helper Banner">
+</p>
+
+<p align="center">
+  <strong>Strings module for the encryption_helper package.</strong>
+</p>
+
+<p align="center">
+  <a href="#attributes">Attributes</a> •
+  <a href="#usage">Usage</a> •
+  <a href="#license">License</a>
+</p>
+
+## Attributes
+
+The `strings.py` module provides a collection of string constants used throughout the `encryption_helper` package. These constants include file suffixes for various key and encrypted file types.
+
+- **private_key_suffix**: The file suffix for private key files.
+- **public_key_suffix**: The file suffix for public key files.
+- **encrypted_file_suffix**: The file suffix for encrypted files.
+
+## Usage
+
+This module defines string constants for use in the `encryption_helper` package. These constants help standardize file naming conventions across the package.
+
+### Example
+
+```python
+from encryption_helper.common.strings import private_key_suffix, public_key_suffix, encrypted_file_suffix
+
+print(private_key_suffix)  # Output: private-key.pem
+print(public_key_suffix)   # Output: public_key.pem
+print(encrypted_file_suffix)  # Output: .bin
+```
+
+## License
+
+This project is licensed under the MIT License. See the [LICENSE](../LICENSE) file for details.