Document on how to contribute a new target
Now that ORC has a CONTRIBUTING.md file, and that a lot of excellent work has been done to add AVX support, it will be good to enhance the document with a brief explanation of what is required to add a new target, including the modification of generate_xml_table.c. This will help us update the target table at https://gstreamer.pages.freedesktop.org/orc/docs/latest/orc-opcodes.html. If a target has poor coverage, it should be include too to know where to improve ORC.