Add explanations for Multi-Line Mode and regular expressions

[catatsuy]

This pull request includes updates to the README.md file to provide more detailed explanations and examples of using regular expressions with Multi-Line Mode in Purl. The most important changes include adding sections on Multi-Line Mode, special cases for empty lines, and practical use cases.

Enhancements to documentation:

• Added a section on Regular Expressions and Multi-Line Mode in Purl, explaining how ^ and ` behave in this mode and why it is useful for line-based text processing.
• Included a special case explanation for matching empty lines, highlighting the difference between lines with zero characters and lines containing only a newline character (\n).
• Added examples of how to use Purl with Multi-Line Mode for common tasks such as filtering lines starting with a specific word, excluding empty lines, and replacing text on specific lines.
• Clarified why the -exclude '^ command does not work for lines that only contain a newline character and provided the correct pattern to use instead.