Add doc for kubelogin and Active Directory ingtegration sample.

Documentation/connectors/kubelogin-activedirectory.md

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+# Integration kubelogin and Active Directory
+
+## Overview
+
+kubelogin is helper tool for kubernetes and oidc integration.
+It makes easy to login Open ID Provider.
+This document describes how dex work with kubelogin and Active Directory.
+
+examples/config-ad-kubelogin.yaml is sample configuration to integrate Active Directory and kubelogin.
+
+## Precondition
+
+1. Active Directory
+You should have Active Directory or LDAP has Active Directory compatible schema such as samba ad.
+You may have user objects and group objects in AD. Please ensure TLS is enabled.
+
+2. Install kubelogin
+Download kubelogin from https://github.com/int128/kubelogin/releases.
+Install it to your terminal.
+
+## Getting started
+
+### Generate certificate and private key
+
+Create OpenSSL conf req.conf as follow:
+
+```
+[req]
+req_extensions = v3_req
+distinguished_name = req_distinguished_name
+
+[req_distinguished_name]
+
+[ v3_req ]
+basicConstraints = CA:FALSE
+keyUsage = nonRepudiation, digitalSignature, keyEncipherment
+subjectAltName = @alt_names
+
+[alt_names]
+DNS.1 = dex.example.com
+```
+
+Please replace dex.example.com to your favorit hostname.
+Generate certificate and private key by following command.
+
+```
+$ openssl req -new -x509 -sha256 -days 3650 -newkey rsa:4096 -extensions v3_req -out openid-ca.pem -keyout openid-key.pem -config req.cnf -subj "/CN=kube-ca" -nodes
+$ ls openid*
+openid-ca.pem openid-key.pem
+```
+
+### Modify dex config
+
+Modify following host, bindDN and bindPW in examples/config-ad-kubelogin.yaml.
+
+```
+connectors:
+- type: ldap
+  name: OpenLDAP
+  id: ldap
+  config:
+    host: ldap.example.com:636
+
+    # No TLS for this setup.
+    insecureNoSSL: false
+    insecureSkipVerify: true
+
+    # This would normally be a read-only user.
+    bindDN: cn=Administrator,cn=users,dc=example,dc=com
+    bindPW: admin0!
+```
+
+### Run dex
+
+```
+$ bin/dex serve examples/config-ad-kubelogin.yaml
+```
+
+### Configure kubernetes with oidc
+
+Copy openid-ca.pem to /etc/ssl/certs/openid-ca.pem on master node.
+
+Use the following flags to point your API server(s) at dex. `dex.example.com` should be replaced by whatever DNS name or IP address dex is running under.
+
+```
+--oidc-issuer-url=https://dex.example.com:32000/dex
+--oidc-client-id=kubernetes
+--oidc-ca-file=/etc/ssl/certs/openid-ca.pem
+--oidc-username-claim=email
+--oidc-groups-claim=groups
+```
+
+Then restart API server(s).
+
+
+See https://kubernetes.io/docs/reference/access-authn-authz/authentication/ for more detail.
+
+### kubelogin
+
+Create context for dex authentication:
+
+```
+$ kubectl config set-context oidc-ctx --cluster=cluster.local --user=test
+$ kubectl config set-credentials test 
+  --auth-provider=oidc \
+  --auth-provider-arg=idp-issuer-url=https://dex.example.com:32000/dex \
+  --auth-provider-arg=client-id=kubernetes \
+  --auth-provider-arg=client-secret=ZXhhbXBsZS1hcHAtc2VjcmV0 \
+  --auth-provider-arg=idp-certificate-authority-data=$(base64 -w 0 openid-ca.pem) \
+  "--auth-provider-arg=extra-scopes=offline_access openid profile email group"
+$ kubectl config use-context oidc-ctx
+```
+
+Please confirm idp-issuer-url, cleint-id, client-secret and idp-certificate-authority-data value is same as config-ad-kubelogin.yaml's value.
+
+Then run kubelogin:
+
+```
+$ kubelogin
+```
+
+Access http://localhost:8000 by web browser and login with your AD account(eg. test@example.com) and password.
+After login and grant, you have following token in ~/.kube/config:
+
+```
+        id-token: eyJhbGciOiJSUzICuU4dCcilDDWlw2lfr8mg...
+        refresh-token: ChlxY2EzeGhKEB4492EzecdKJOElECK...
+```
+