Added security note to README.md

Author: sybrenstuvel

README.md

@@ -23,6 +23,12 @@ or download it from the [Python Package Index](https://pypi.org/project/rsa/).
 The source code is maintained at [GitHub](https://github.com/sybrenstuvel/python-rsa/) and is
 licensed under the [Apache License, version 2.0](https://www.apache.org/licenses/LICENSE-2.0)
 
+Security
+--------
+
+Because of how Python internally stores numbers, it is very hard (if not impossible) to make a pure-Python program secure against timing attacks. This library is no exception, so use it with care.
+
+
 Major changes in 4.1
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