Add simple usage doc for accessing secret/configmap in function (#484)

Documentation/docs-site/content/tutorial/_index.en.md

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+---
+title: "Tutorials"
+weight: 40
+---
+
+* [Access Secret/ConfigMap In Function](./access-secret-cfgmap-in-function)

Documentation/docs-site/content/tutorial/access-secret-cfgmap-in-function.md

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+---
+title: "Access secret/configmap in function"
+draft: false
+weight: 41
+---
+
+From fission v0.5.0 and later, functions are able to access [Secrets](https://kubernetes.io/docs/concepts/configuration/secret/) and [ConfigMaps](https://kubernetes.io/docs/concepts/storage/volumes/#configmap) specified by users.
+
+### Create Secret and ConfigMap
+
+You can create Secret and ConfigMap with CLI.
+
+``` bash
+$ kubectl -n default create secret generic foo --from-literal=TEST_KEY="TESTVALUE"
+$ kubectl -n default create configmap bar --from-literal=TEST_KEY=TESTVALUE
+```
+
+Or use `kubectl create -f <filename.yaml>` to create these from a YAML file.
+
+``` yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  namespace: default
+  name: foo
+data:
+  TEST_KEY: VEVTVFZBTFVF # value after base64 encode
+type: Opaque
+
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  namespace: default
+  name: bar
+data:
+  TEST_KEY: TESTVALUE
+```
+
+### Access Secret and ConfigMap
+
+Since content of Secret and ConfigMap are key-value pairs, functions can access them with following paths:
+
+``` bash
+# Secret path
+/secrets/<namespace>/<name>/<key>
+
+# ConfigMap path
+/configs/<namespace>/<name>/<key>
+```
+
+From the previous example, the paths are:
+
+``` bash
+# secret foo
+/secrets/default/foo/TEST_KEY
+
+# confimap bar
+/configs/default/bar/TEST_KEY
+```
+
+Now, let's create a simple python function (leaker.py) that return value of Secret `foo` and ConfigMap `bar`.
+
+``` python
+# leaker.py
+
+def main():
+    path = "/configs/default/bar/TEST_KEY"
+    f = open(path, "r")
+    config = f.read()
+
+    path = "/secrets/default/foo/TEST_KEY"
+    f = open(path, "r")
+    secret = f.read()
+
+    msg = "ConfigMap: %s\nSecret: %s" % (config, secret)
+
+    return msg, 200
+```
+
+
+Create environment, function and http trigger.
+
+``` bash
+# create python env
+$ fission env create --name python --image fission/python-env
+
+# create function named "leaker"
+$ fission fn create --name leaker --env python --code leaker.py --secret foo --configmap bar
+
+# create route(http trigger)
+$ fission route create --function leaker --url /leaker --method GET
+```
+
+
+Try to access the function, the output should look like following.
+
+``` bash
+$ curl http://$FISSION_ROUTER/leaker
+ConfigMap: TESTVALUE
+Secret: TESTVALUE
+```
+
+Note: If the Secret or ConfigMap value is updated, the function may not get the updated value for some time; it may get a cached older value.
+