update readme

* document username/password
* document pushing support
* update intro text, reference concourse/builder

README.md

@@ -1,7 +1,6 @@
 # Registry Image Resource
 
-Supports checking, fetching, and (eventually) pushing of images to Docker
-registries.
+Supports checking, fetching, and pushing of images to Docker registries.
 
 This resource is intended as a replacement for the [Docker Image
 resource](https://github.com/concourse/docker-image-resource). Here are the key
@@ -9,26 +8,35 @@ differences:
 
 * This resource is implemented in pure Go and does not use the Docker daemon or
   CLI.
+
+* This resource has stronger test coverage.
+
 * Therefore, it does not need to run privileged, and should be much more
   efficient. It will also be less error-prone (parsing `docker` CLI output was
   janky).
-* This resource does *not* support building. This should instead be done with a
-  task. Using [Kaniko](https://github.com/GoogleContainerTools/kaniko) would be
-  a great idea!
-* A goal of this resource is to stay as small and simple as possible. The
-  Docker Image resource grew way too large and complicated. Initially, it this
-  resource is designed solely to support being used to fetch images for
-  Concourse containers (using `image_resource` and `resource_types`).
-  * This may need to expand later on as it would be completely reasonable to
-    implement `put` here, so we may need to be able to `get` in a different
-    format in order to be symmetrical.
-* This resource has stronger test coverage.
+
+* This resource does *not* support building. This should instead be done with
+  something like the [`concourse/builder`
+  task](https://github.com/concourse/builder) (or anything that can produce OCI
+  image tarballs).
+
+* A goal of this resource is to stay as focused and simple as possible. The
+  Docker Image resource grew way too large and complicated. There are simply
+  too many ways to build and publish Docker images. It will be easier to
+  support many smaller resources + tasks rather than one huge interface.
 
 
 ## Source Configuration
 
 * `repository`: *Required.* The name of the repository, e.g. `alpine`.
-* `tag`: *Optional. Default `latest`.* The name of the tag to monitor.
+
+* `tag`: *Optional. Default `latest`.* The name of the tag to monitor and
+  publish to.
+
+* `username` and `password`: *Optional.* A username and password to use when
+  authenticating to the registry. Must be specified for private repos or when
+  using `put`.
+
 * `debug`: *Optional. Default `false`.* If set, progress bars will be disabled
   and debugging output will be printed instead.
 
@@ -37,15 +45,18 @@ differences:
 
 ### `check`: Discover new digests.
 
-Reports the current digest that the registry has for the given tag.
+Reports the current digest that the registry has for the tag configured in
+`source`.
+
 
 ### `in`: Fetch the image's rootfs and metadata.
 
+Fetches an image.
+
 #### Parameters
 
 * `format`: *Optional. Default `rootfs`.* The format to fetch as. (See below.)
 
-
 #### Formats
 
 ##### `rootfs`
@@ -72,6 +83,11 @@ In this format, the resource will produce the following files:
 
 ### `out`: Push an image up to the registry under the given tags.
 
-**Not implemented yet.** Once implemented, this may take an image in a standard
-format (say, whatever `docker save` does) and upload it to the registry to the
-tag configured in `source`.
+Uploads an image to the registry under the tag configured in `source`.
+
+The currently encouraged way to build these images is by using the
+[`concourse/builder` task](https://github.com/concourse/builder).
+
+#### Parameters
+
+* `image`: *Required.* The path to the OCI image tarball to upload.