Add path validation error expectations for PKITS tests.

net/data/verify_certificate_chain_unittest/README

 This directory contains test data for verifying certificate chains.
 
 Tests are grouped into directories that contain the keys, python to generate
 chains, and test expectations. "DIR" is used as a generic placeholder below to
 identify such a directory.
 
 ===============================
 DIR/generate-chains.py
 ===============================
 
@@ skipping 53 matching lines @@
       "CLIENT_AUTH"
 
   "errors" - This has special parsing rules: it is interpreted as the
       final key in the file. All lines after "errors:\n" are read as being the
       error string (this allows embedding newlines in it).
 
 Additionally, it is possible to add python-style comments by starting a line
 with "#".
 
 ===============================
+pkits_errors/*.txt
+===============================
+
+These files contain the expected errors for PKITS tests
+(third_party/nist-pkits). The file name correspond so the PKITS tests number.
+They are baselined specifically for VerifyCertificateChain().
+
+===============================
 generate-all.sh
 ===============================
 
 Runs all of the generate-chains.py scripts and cleans up the temp files
 afterwards.