Add readme on login/logout (oras-project#60)

* Add readme on login/logout

* Remove registry tag refs in readme

README.md

@@ -29,7 +29,7 @@ The simplest way to get started is to run the official
 [Docker registry image](https://hub.docker.com/_/registry) locally:
 
 ```
-docker run -it --rm -p 5000:5000 registry:2.7.0
+docker run -it --rm -p 5000:5000 registry
 ```
 
 This will start a Distribution server at `localhost:5000`
@@ -99,14 +99,42 @@ docker run -it --rm -v $(pwd):/workspace orasbot/oras:v0.3.3 help
 
 Note: the default WORKDIR  in the image is `/workspace`.
 
-### Login Credentials
-`oras` uses the local Docker credentials by default. Please run `docker login` in advance for any private registries.
+### Authentication
+
+Run `oras login` in advance for any private registries. By default, this will store credentials in `~/.docker/config.json` (same file as used by Docker). If you have authenticated to a registry previously using `docker login`, the credentials will be reused. Use the `-c`/`--config` option to specify an alternate location.
 
 `oras` also accepts explicit credentials via options, for example,
 ```
 oras pull -u username -p password myregistry.io/myimage:latest
 ```
 
+#### Example using with Docker registry
+
+First, create a valid htpasswd file (must use `-B` for bcrypt):
+```
+htpasswd -cB -b auth.htpasswd myuser mypass
+```
+
+Next, start a registry using that file for auth:
+```
+docker run -it --rm -p 5000:5000 \
+    -v $(pwd)/auth.htpasswd:/etc/docker/registry/auth.htpasswd \
+    -e REGISTRY_AUTH="{htpasswd: {realm: localhost, path: /etc/docker/registry/auth.htpasswd}}" \
+    registry
+```
+
+In a new window, login with `oras`:
+```
+oras login -u myuser -p mypass localhost:5000
+```
+
+You will notice a new entry for `localhost:5000` appear in `~/.docker/config.json`.
+
+To remove the entry from the credentials file, use `oras logout`:
+```
+oras logout localhost:5000
+```
+
 ### Usage
 
 #### Pushing single files to remote registry