add example to README

README.md

@@ -1,2 +1,50 @@
 # chartpress
-automate building and publishing images for helm charts
+
+Automate building and publishing helm charts and associated images.
+
+This is used as part of the JupyterHub and Binder projects.
+
+Chartpress will:
+
+- build docker images and tag them with the latest git commit
+- publish those images to DockerHub
+- rerender a chart to include the tagged images
+- publish the chart and index to gh-pages
+
+A `chartpress.yaml` file contains a specification of charts and images to build.
+
+For example:
+
+```yaml
+charts:
+  # list of charts by name
+  # each name should be a directory containing a helm chart
+  - name: binderhub
+    # the prefix to use for built images
+    imagePrefix: jupyterhub/k8s-
+    # the git repo whose gh-pages contains the charts
+    repo:
+      git: jupyterhub/helm-chart
+      published: https://jupyterhub.github.io/helm-chart
+    # additional paths (if any) relevant to the chart version
+    # outside the chart directory itself
+    paths:
+      - ../setup.py
+      - ../binderhub
+    # images to build
+    images:
+      binderhub:
+        # Context to send to docker build for use by the Dockerfile
+        # (if different from the current directory)
+        contextPath: ..
+        # Dockerfile path, if different from the default
+        # (may be needed if contextPath is set)
+        dockerfilePath: images/binderhub/Dockerfile
+        # path in values.yaml to be updated with image name and tag
+        valuesPath: image
+        # additional paths (if any) relevant to the image
+        # outside the image directory itself
+        paths:
+          - ../setup.py
+          - ../binderhub
+```