Added documentation about stringData (#10938)

* Added documentation about stringData

I've added some documentation about the stringData field, including a simple example showing how it can be used.

* fixup! Added documentation about stringData

* fixup! Added documentation about stringData

content/en/docs/concepts/configuration/secret.md

@@ -102,19 +102,24 @@ See [decoding a secret](#decoding-a-secret) for how to see the contents.
 
 #### Creating a Secret Manually
 
-You can also create a secret object in a file first,
-in json or yaml format, and then create that object.
+You can also create a Secret in a file first, in json or yaml format,
+and then create that object. The
+[Secret](/docs/reference/generated/kubernetes-api/v1.12/#secret-v1-core) contains two maps:
+data and stringData. The data field is used to store arbitrary data, encoded using
+base64. The stringData field is provided for convenience, and allows you to provide
+secret data as unencoded strings.
 
-Each item must be base64 encoded:
+For example, to store two strings in a Secret using the data field, convert
+them to base64 as follows:
 
 ```shell
-$ echo -n 'admin' | base64
+echo -n 'admin' | base64
 YWRtaW4=
-$ echo -n '1f2d1e2e67df' | base64
+echo -n '1f2d1e2e67df' | base64
 MWYyZDFlMmU2N2Rm
 ```
 
-Now write a secret object that looks like this:
+Write a Secret that looks like this:
 
 ```yaml
 apiVersion: v1
@@ -127,15 +132,108 @@ data:
   password: MWYyZDFlMmU2N2Rm
 ```
 
-The data field is a map.  Its keys must consist of alphanumeric characters, '-', '_' or '.'.  The values are arbitrary data, encoded using base64.
-
-Create the secret using [`kubectl create`](/docs/reference/generated/kubectl/kubectl-commands#create):
+Now create the Secret using [`kubectl create`](/docs/reference/generated/kubectl/kubectl-commands#create):
 
 ```shell
 $ kubectl create -f ./secret.yaml
 secret "mysecret" created
 ```
 
+For certain scenarios, you may wish to use the stringData field instead. This
+field allows you to put a non-base64 encoded string directly into the Secret,
+and the string will be encoded for you when the Secret is created or updated.
+
+A practical example of this might be where you are deploying an application
+that uses a Secret to store a configuration file, and you want to populate
+parts of that configuration file during your deployment process.
+
+If your application uses the following configuration file:
+
+```yaml
+apiUrl: "https://my.api.com/api/v1"
+username: "user"
+password: "password"
+```
+
+You could store this in a Secret using the following:
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: mysecret
+type: Opaque
+stringData:
+  config.yaml: |-
+    apiUrl: "https://my.api.com/api/v1"
+    username: {{username}}
+    password: {{password}}
+```
+
+Your deployment tool could then replace the `{{username}}` and `{{password}}`
+template variables before running `kubectl create`.
+
+stringData is a write-only convenience field. It is never output when
+retrieving Secrets. For example, if you run the following command:
+
+```shell
+kubectl get secret mysecret -o yaml
+```
+
+The output will be similar to:
+
+```yaml
+apiVersion: v1
+data:
+  config.yaml: YXBpVXJsOiAiaHR0cHM6Ly9teS5hcGkuY29tL2FwaS92MSIKdXNlcm5hbWU6IHt7dXNlcm5hbWV9fQpwYXNzd29yZDoge3twYXNzd29yZH19
+kind: Secret
+metadata:
+  creationTimestamp: 2018-11-15T20:40:59Z
+  name: mysecret
+  namespace: default
+  resourceVersion: "7225"
+  selfLink: /api/v1/namespaces/default/secrets/mysecret
+  uid: c280ad2e-e916-11e8-98f2-025000000001
+type: Opaque
+```
+
+If a field is specified in both data and stringData, the value from stringData
+is used. For example, the following Secret definition:
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: mysecret
+type: Opaque
+data:
+  username: YWRtaW4=
+stringData:
+  username: administrator
+```
+
+Results in the following secret:
+
+```yaml
+apiVersion: v1
+data:
+  username: YWRtaW5pc3RyYXRvcg==
+kind: Secret
+metadata:
+  creationTimestamp: 2018-11-15T20:46:46Z
+  name: mysecret
+  namespace: default
+  resourceVersion: "7579"
+  selfLink: /api/v1/namespaces/default/secrets/mysecret
+  uid: 91460ecb-e917-11e8-98f2-025000000001
+type: Opaque
+```
+
+Where `YWRtaW5pc3RyYXRvcg==` decodes to `administrator`.
+
+The keys of data and stringData must consist of alphanumeric characters,
+'-', '_' or '.'.
+
 **Encoding Note:** The serialized JSON and YAML values of secret data are
 encoded as base64 strings.  Newlines are not valid within these strings and must
 be omitted.  When using the `base64` utility on Darwin/macOS users should avoid