Updated README.md

Author: str4d

README.md

@@ -5,6 +5,8 @@ This is an implementation of Ed25519 in Java. Structurally, it is based on the r
 
 There are no guarantees that this is secure for use. Tests against [the data from the Python implementation](http://ed25519.cr.yp.to/python/sign.input) are passing, but this has not yet been audited by a professional cryptographer. In particular, this implementation is unlikely to have the constant-time properties of ref10 (for now).
 
+The code requires Java 6 (for e.g. the `Arrays.copyOfRange()` calls in `EdDSAEngine.engineVerify()`).
+
 The JUnit4 tests require the Hamcrest library `hamcrest-all.jar`.
 
 This code is released to the public domain and can be used for any purpose.