chore: enhanced app install docu

Author: mgoltzsche

CONTRIBUTING.md

@@ -1,3 +1,5 @@
 # Contributing to Jenkins X documentation
 
-Please visit the contributing guide for the documentation available at [Jenkins X website](https://jenkins-x.io/docs/contributing/)
+Please visit the contributing guide for the documentation available at [Jenkins X website](https://jenkins-x.io/docs/contributing/).  
+
+In order to see local changes to the navigation applied please restart your local hugo preview server since hugo does not reload all metadata on-the-fly.

content/en/docs/labs/enhancements

@@ -3,4 +3,6 @@ title: "Extending Jenkins X"
 linkTitle: "Extending Jenkins X"
 description: Learn how to extend Jenkins X using the Apps Framework
 weight: 12
----
+aliases:
+  - /docs/guides/extending-jx/
+---

content/en/docs/resources/guides/extending-jx/_index.md

@@ -3,7 +3,7 @@ title: Introduction
 linktitle: Apps Framework
 description: Extending Jenkins X using the Apps Framework
 type: docs
-weight: 1
+weight: 12
 aliases:
     - /docs/contributing/addons/
     - /docs/guides/extending-jx/introduction/
@@ -57,6 +57,35 @@ Planned features include:
 * `jx get apps` - the ability to list all apps that can be installed
 * integration for bash completion
 
+## Structure of an App chart
+
+In addition to the structure of a regular helm chart a Jenkins X App chart contains the following:
+* `values.schema.yaml`: a JSON schema extended with conditions, field values and questions that is used to point out and prompt the user for customizable values the app should be installed with.
+* `templates/app.yaml`: a Jenkins X `App` custom resource: allows to define a `values.schema.yaml` transformation which can be used to inject cluster-specific values as for instance its domain and TLS settings (which can be obtained by mounting the ConfigMap `ingress-config`).
+
+When `jx add app` is called the corresponding helm chart is fetched, its `values.schema.yaml` transformed using the `App` resource and the user prompted for the contained questions (non-const fields).
+From the user's answers a `values.yaml` is generated and used to install the chart.
+
+## How to test local App chart changes
+
+You can build your app chart locally, upload it in your cluster's chart repository and call `jx add app` referencing your uploaded chart.
+This can be done as follows from within your chart's directory:
+```
+CHART_NAME=<YOUR_APP_NAME>
+CHART_VERSION=<YOUR_APP_VERSION>
+CHART_REPO=<YOUR_CHART_REPO_URL> # listed by `jx get urls`
+CHART_REPO_USR=<YOUR_USERNAME>
+CHART_REPO_PSW=<YOUR_PASSWORD>
+helm init --client-only
+helm repo add <DEP_REPO_NAME> <DEP_REPO_URL> # required if dependencies need to be loaded
+helm dependency build
+sed "s/version: .*/version: $CHART_VERSION/g" -i Chart.yaml # set the app's current version
+helm lint .
+helm package .
+curl --fail -u "$CHART_REPO_USR:$CHART_REPO_PSW" --data-binary "@${CHART_NAME}-${CHART_VERSION}.tgz" ${CHART_REPO}/api/charts
+jx add app --repository $CHART_REPO --version $CHART_VERSION $CHART_NAME
+```
+
 # APIs
 
 APIs can be consumed from a microservice deployed to Kubernetes, from outside the cluster (with appropriate