Broken links in README file

Links to list of supported flash devices (https://www.xmos.com/support/knowledgebase/silicon/which-flash-devices-are-supported-xmos) and link to SPI spec files for some unsupported devices (https://www.xcore.com/wiki/index.php/SPI_spec_files_for_use_with_xflash_and_flashlib) in README file are broken.