secret.md: add base64 risk (#3791)

* secret.md: add base64 risk

Solves #3681.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

* Update secret.md

docs/concepts/configuration/secret.md

@@ -805,6 +805,10 @@ Pod level](#use-case-secret-visible-to-one-container-in-a-pod).
    - Administrators should limit access to etcd to admin users
    - Secret data in the API server is at rest on the disk that etcd uses; admins may want to wipe/shred disks
      used by etcd when no longer in use
+ - If you configure the secret through a manifest (JSON or YAML) file which has
+   the secret data encoded as base64, sharing this file or checking it in to a
+   source repository means the secret is compromised. Base64 encoding is not an
+   encryption method and is considered the same as plain text.
  - Applications still need to protect the value of secret after reading it from the volume,
    such as not accidentally logging it or transmitting it to an untrusted party.
  - A user who can create a pod that uses a secret can also see the value of that secret.  Even