[doc] readme: update SIFT patent description

Author: fabiencastan

README.md

@@ -109,7 +109,9 @@ We acknowledge that there is at least one SIFT implementation that is vastly fas
 ## License
 
 PopSift is licensed under [MPL v2 license](COPYING.md).
-However, SIFT is patented in the US and perhaps other countries, and this license does not release users of this code from any requirements that may arise from such patents.
+SIFT was patented in the United States from 1999-03-08 to 2020-03-28. See the [patent link](https://patents.google.com/patent/US6711293B1/en) for more information.
+This license does not release users of this code from any requirements that may arise from such patents.
+
 
 ## Cite Us