Show example usage of both tools

Signed-off-by: Josh Dolitsky <josh@dolit.ski>

README.md

@@ -18,19 +18,39 @@ The following third-party Docker credential helpers are currently supported:
 
 ## Installation
 
-Download [latest release](https://github.com/jdolitsky/docker-credential-magic/releases/latest).
-
-Install manually:
+Download [latest release](https://github.com/jdolitsky/docker-credential-magic/releases/latest) tarball
+for your system and install both tools manually:
 
 ```
-go install github.com/jdolitsky/docker-credential-magic/cmd/docker-credential-magic@latest
-go install github.com/jdolitsky/docker-credential-magic/cmd/docker-credential-magician@latest
+cat docker-credential-magic*.tar.gz | \
+  tar x -C /usr/local/bin 'docker-credential-magic*'
 ```
 
 ## Usage
 
+### `docker-credential-magic`
+
+The following example shows how `docker-credential-magic` can be used to
+proxy auth to `docker-credential-gcr`, based on the detection of a `*.gcr.io` domain:
+
+*Note: Example requires [`docker-credential-gcr`](https://github.com/GoogleCloudPlatform/docker-credential-gcr)
+to be pre-installed*
+
+```
+$ export GOOGLE_APPLICATION_CREDENTIALS="${PWD}/service-account-key.json"
+```
+
+```
+$ echo "us.gcr.io" | docker-credential-magic get
+{"ServerURL":"us.gcr.io","Username":"_dcgcr_token","Secret":"*****"}
+```
+
 ### `docker-credential-magician`
 
+The following example shows how `docker-credential-magician` can be used to
+augment the [`cosign`](https://github.com/sigstore/cosign) image with
+various credential helpers, and set the default credential store to `magic`:
+
 *Note: Requires local Docker daemon to be running*
 
 ```
@@ -41,12 +61,15 @@ $ docker-credential-magician gcr.io/projectsigstore/cosign/ci/cosign:v0.5.0
 ```
 $ docker run --rm --entrypoint sh \
     gcr.io/projectsigstore/cosign/ci/cosign:v0.5.0.magic \
-    -c 'ls -lah /opt/magic/bin'
-total 24M
+    -c 'ls -lah /opt/magic/bin &&
+        env | grep DOCKER_CONFIG &&
+        cat $DOCKER_CONFIG/config.json'
 drwxr-xr-x    2 root     root        4.0K Jul 19 19:49 .
 drwxr-xr-x    3 root     root        4.0K Jul 19 19:49 ..
 -r-xr-xr-x    1 root     root        8.7M Jan  1  1970 docker-credential-acr-env
 -r-xr-xr-x    1 root     root        7.8M Jan  1  1970 docker-credential-ecr-login
 -r-xr-xr-x    1 root     root        5.6M Jan  1  1970 docker-credential-gcr
 -r-xr-xr-x    1 root     root        2.2M Jan  1  1970 docker-credential-magic
+DOCKER_CONFIG=/opt/magic
+{"credsStore":"magic"}
 ```