Merge pull request opencv#19248 from LaurentBerger:forum

Author: alalek

README.md

@@ -4,7 +4,8 @@
 
 * Homepage: <http://opencv.org>
 * Docs: <http://docs.opencv.org/3.4/>
-* Q&A forum: <http://answers.opencv.org>
+* Q&A forum: <https://forum.opencv.org>
+  * previous forum (read only): <http://answers.opencv.org>
 * Issue tracking: <https://github.com/opencv/opencv/issues>
 
 ### Contributing