feat: add custom requirements file support

- Add -f/--requirements-file option to specify custom requirements file paths
- Enhance requirements file detection with clear user guidance
- Show available files and usage example when multiple files found
- Update documentation with examples and troubleshooting guidance
- Improve error handling for custom file paths

README.md

@@ -14,6 +14,8 @@ Compatible with Python 3.11, 3.12, and 3.13.
 - Clean, informative output with version information
 - Preserves requirements.txt comments and formatting
 - Creates requirements.txt if it doesn't exist
+- Support for custom requirements file paths
+- Smart detection of multiple requirements files
 - Full support for Python 3.11, 3.12, and 3.13
 
 ## Installation
@@ -53,6 +55,34 @@ pip-add -d requests
 #   - requests (2.32.3)
 #   - urllib3 (2.2.3)
 # ✓ Updated requirements.txt
+
+# Install using custom requirements file
+pip-add -f requirements/dev.txt requests
+# Output:
+# Installing requests...
+# ✓ Successfully installed requests (2.32.3)
+# ✓ Updated requirements/dev.txt
+```
+
+### Multiple Requirements Files
+
+When multiple requirements files are found in your project:
+
+```bash
+# Tool will show available files:
+pip-add requests
+# Output:
+# ℹ️  Found multiple requirements files:
+#   - requirements.txt
+#   - requirements/dev.txt
+#   - requirements/prod.txt
+#
+# Using: requirements.txt
+# To use a specific file, run the command with -f/--requirements-file option:
+# Example: pip-add -f requirements/dev.txt requests
+
+# Specify which file to use:
+pip-add -f requirements/dev.txt requests
 ```
 
 ### Removal
@@ -79,12 +109,19 @@ pip-add -d -r requests
 #   - idna (needed by: email-validator, cryptography)
 #
 # ✓ Updated requirements.txt
+
+# Remove from specific requirements file
+pip-add -r -f requirements/dev.txt requests
+# Output:
+# Removing packages...
+# ✓ Successfully uninstalled requests (2.32.3)
+# ✓ Updated requirements/dev.txt
 ```
 
 ## Command Line Options
 
 ```
-pip-add [-h] [-d] [-e] [-r] package
+pip-add [-h] [-d] [-e] [-r] [-f REQUIREMENTS_FILE] package
 
 positional arguments:
   package               Package to install or remove
@@ -94,6 +131,8 @@ options:
   -d, --dependencies   Include dependencies when installing or removing
   -e, --exact         Use == instead of >= for version specification
   -r, --remove        Remove package(s) and their entries from requirements.txt
+  -f, --requirements-file
+                      Path to custom requirements.txt file
 ```
 
 ## How It Works
@@ -103,7 +142,7 @@ options:
 1. Installs the specified package using pip
 2. Retrieves installed version information
 3. With `-d`: tracks and installs all dependencies
-4. Updates requirements.txt with new package(s)
+4. Updates requirements.txt (or specified requirements file) with new package(s)
 5. Uses `>=` by default or `==` with `-e` flag
 
 ### Removal Process
@@ -112,9 +151,21 @@ options:
 2. Identifies which dependencies are safe to remove
 3. Checks if any dependencies are needed by other packages
 4. Safely removes unused packages
-5. Updates requirements.txt
+5. Updates requirements.txt (or specified requirements file)
 6. Reports kept dependencies and their dependents
 
+### Requirements File Handling
+
+1. By default, looks for requirements.txt in the current directory
+2. Creates requirements.txt if it doesn't exist
+3. With `-f`: uses specified requirements file path
+4. Creates directories if needed for custom file paths
+5. Preserves comments and formatting in existing files
+6. When multiple files are found:
+   - Lists all available requirements files
+   - Shows which file will be used by default
+   - Provides example command to specify a particular file
+
 ## Safe Dependency Handling
 
 The tool is designed to safely handle dependencies:
@@ -155,6 +206,11 @@ pip install -e .
 # First time setup
 pip-add -d flask
 # Creates requirements.txt and adds Flask with dependencies
+
+# Multiple requirements files
+pip-add -f requirements/dev.txt pytest
+pip-add -f requirements/prod.txt gunicorn
+# Manages separate requirement files for different environments
 ```
 
 ### Updating Dependencies
@@ -178,6 +234,7 @@ pip-add -d -r flask
 1. **Package not found in requirements.txt**
    - The file will be created automatically
    - Existing comments are preserved
+   - Use `-f` to specify a different requirements file
 
 2. **Dependency conflicts**
    - Uses `>=` by default to minimize conflicts
@@ -186,3 +243,9 @@ pip-add -d -r flask
 3. **Dependencies not removing**
    - Check the output for dependencies kept
    - Tool will show which packages need them
+
+4. **Multiple requirements files**
+   - Tool will list all available requirements files
+   - Shows which file will be used by default
+   - Provides example command to specify a particular file
+   - Use `-f` to specify which file to use