doc: add note for xl vendor prefix

Signed-off-by: Huaqi Fang <[email protected]>

Author: fanghuaqi

README.mkd

@@ -301,7 +301,8 @@ T-Head                 | th              | https://www.t-head.cn/
 Ventana Micro Systems  | vt              | https://www.ventanamicro.com/
 Nuclei                 | xl              | https://nucleisys.com/
 
-NOTE: Vendor prefixes are case-insensitive.
+- NOTE: Vendor prefixes are case-insensitive.
+- NOTE: The Nuclei instruction prefix `xl` is an abbreviation of "XinLai", which is the Chinese pronunciation of Nuclei(芯来).
 
 ### List of vendor extensions