📣 borb release notes

• add Watermark LayoutElement, representing a piece of text laid out over the entire Page. Users can specify the text, font_size, font_color, rotation, and transparency
• add Watermark to easy imports
• add tests for Watermark
• re-implement MultiColumnLayout, with better handling of headers and footers
• update requirements.txt where possible
• reworked a couple of tests, ensuring they can run without dependencies as well
• added a fair use warning to ensure people know the difference between using borb in an open-source vs. commercial setting. This warning is only triggered when producing a high volume of PDF documents.