Document PKINIT and anonymos PKINIT configuration

(cherry picked from commit eda5048bb5b0bbf4deea08830551d3578f223059)

https://github.com/krb5/krb5/commit/d708838e7bacf165d3c86adb2ea0c560803d0ede
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: d708838e7bacf165d3c86adb2ea0c560803d0ede
Branch: krb5-1.11
 doc/admins/index.rst  |    1 +
 doc/admins/pkinit.rst |  219 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 220 insertions(+), 0 deletions(-)