Update external test README for running krb5

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2022)

test/README.external

@@ -98,3 +98,37 @@ Test failures and suppressions
 Some tests target older (<=1.0.2) versions so will not run. Other tests target
 other crypto implementations so are not relevant. Currently no tests fail.
 
+
+krb5 test suite
+===============
+
+Much like the PYCA/Cryptography test suite, this builds and runs the krb5
+tests against the local OpenSSL build.
+
+You will need a git checkout of krb5 at the top level:
+
+$ git clone https://github.com/krb5/krb5
+
+krb5's master has to pass this same CI, but a known-good version is
+krb5-1.15.1-final if you want to be sure.
+
+$ cd krb5
+$ git checkout krb5-1.15.1-final
+$ cd ..
+
+OpenSSL must be built with external tests enabled:
+
+$ ./config enable-external-tests
+$ make
+
+krb5's tests will then be run as part of the rest of the suite, or can be
+explicitly run (with more debugging):
+
+$ VERBOSE=1 make TESTS=test_external_krb5 test
+
+Test failures supressions
+-------------------------
+
+krb5 will automatically adapt its test suite to account for the configuration
+of your system.  Certain tests may require more installed packages to run.  No
+tests are expected to fail.