Save the private keys used by generated verify_certificate_chain tests.

net/data/verify_certificate_chain_unittest/README

Index: net/data/verify_certificate_chain_unittest/README
diff --git a/net/data/verify_certificate_chain_unittest/README b/net/data/verify_certificate_chain_unittest/README
index a529a508eff9a2cf9216160a1c32e3b7e0b1536a..87a46987990f19acbb63f9f1afe99e9294f9fa45 100644
--- a/net/data/verify_certificate_chain_unittest/README
+++ b/net/data/verify_certificate_chain_unittest/README
@@ -17,6 +17,15 @@ generate-all.sh
 Runs all of the generate-*.py scripts and does some cleanup.
 
 ===============================
+keys/XXX/*.key
+===============================
+
+The keys used/generated by test XXX. The private keys shouldn't be needed to run
+the tests, however are useful when re-generating the test data to have stable
+results (at least for signature types which are deterministic, like RSASSA
+PKCS#1 which is used by most of the certificates data).
+
+===============================
 *.pem
 ===============================