Add README.md file to rsabigexponent system test

This README.md describes why is bigkey needed.

bin/tests/system/rsabigexponent/README.md

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+Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+
+See COPYRIGHT in the source root or https://isc.org/copyright.html for terms.
+
+The `rsabigexponent` test is used to `check max-rsa-exponent-size`.
+
+We only run this test on builds without PKCS#11, as we have control over
+the RSA exponent size with plain OpenSSL. We have not explored how to do
+this with PKCS#11, which would require generating such a key and then
+signing a zone with it. Additionally, even with control of the exponent
+size with PKCS#11, generating a DNSKEY with this property and signing
+such a zone would be slow and undesirable for each test run; instead, we
+use a pregenerated DNSKEY and a saved signed zone.  These are located in
+`rsabigexponent/ns2` and currently use RSASHA1 for the `DNSKEY`
+algorithm; however, that may need to be changed in the future.
+
+To generate the `DNSKEY` used in this test, we used `bigkey.c`, as
+dnssec-keygen is not capable of generating such keys.
+
+Do **not** remove `bigkey.c` as it may be needed to generate a new
+`DNSKEY` for testing purposes.
+
+`bigkey` is used to both test that we are not running under PKCS#11 and
+generate a `DNSKEY` key with a large RSA exponent.