SU3 spec: Add link to Python RSA signing how-to

i2p2www/spec/updates.rst

@@ -3,8 +3,8 @@ Software Update Specification
 =============================
 .. meta::
     :category: Design
-    :lastupdated: June 2021
-    :accuratefor: 0.9.51
+    :lastupdated: 2022-02
+    :accuratefor: 0.9.53
 
 .. contents::
 
@@ -300,6 +300,10 @@ the content type is that expected for the application.
 
 All values are in network byte order (big endian).
 
+For a python implementation of Raw RSA signatures compatible with Java "NONEwithRSA",
+see this Stack Overflow article: [Python]_
+
+
 SU3 Router Update File Specification
 ------------------------------------
 
@@ -657,6 +661,9 @@ References
 .. [PLUGIN]
     {{ spec_url('plugin') }}
 
+.. [Python]
+    https://stackoverflow.com/questions/59573121/python-rsa-sign-a-string-with-nonewithrsa/68301530#68301530
+
 .. [RFC-3339]
     https://tools.ietf.org/html/rfc3339