adding short explainers for index pages

docs/source/configuration/index.rst

@@ -2,6 +2,9 @@
 Configuring your repository
 ===========================
 
+Information about configuring your repository to work with repo2docker,
+and controlling elements of the built environment using configuration files.
+
 .. toctree::
    :maxdepth: 2
    :caption: Complete list of configuration files

docs/source/contributing/index.rst

@@ -2,6 +2,11 @@
 Contributing
 ============
 
+The repo2docker community is welcoming of all kinds of help and
+participation from others. Below are a few ways that you can get involved,
+as well as resources for understanding the structure and design of the
+repo2docker package.
+
 .. toctree::
 
    contributing

docs/source/getting-started/index.rst

@@ -2,9 +2,11 @@
 Getting Started
 ===============
 
+Instructions and information on how to get started with repo2docker
+on your own machine. Select from the pages listed below to begin.
+
 .. toctree::
    :maxdepth: 2
-   :caption: Getting started with repo2docker
 
    ../install
    ../usage

docs/source/howto/index.rst

@@ -3,6 +3,7 @@ How-to Guides
 =============
 
 Short, actionable guides that cover specific topics with repo2docker.
+Select from the pages listed below to get started.
 
 .. toctree::
    :maxdepth: 2