Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

README.md

@@ -27,6 +27,8 @@ $ make osxkeychain
 
 ## Usage
 
+### With the Docker Engine
+
 Set the `credsStore` option in your `.docker/config.json` file with the suffix of the program you want to use. For instance, set it to `osxkeychain` if you want to use `docker-credential-osxkeychain`.
 
 ```json
@@ -35,11 +37,24 @@ Set the `credsStore` option in your `.docker/config.json` file with the suffix o
 }
 ```
 
+### With other command line applications
+
+The sub-package [client](https://godoc.org/github.com/docker/docker-credential-helpers/client) includes
+functions to call external programs from your own command line applications.
+
+There are three things you need to know if you need to interact with a helper:
+
+1. The name of the program to execute, for instance `docker-credential-osxkeychain`.
+2. The server address to identify the credentials, for instance `https://example.com`.
+3. The username and secret to store, when you want to store credentials.
+
+You can see examples of each function in the [client](https://godoc.org/github.com/docker/docker-credential-helpers/client) documentation.
+
 ### Available programs
 
 1. osxkeychain: Provides a helper to use the OS X keychain as credentials store.
-1. secretservice: Provides a helper to use the D-Bus secret service as credentials store.
-2. wincred: Provides a helper to use Windows credentials manager as store.
+2. secretservice: Provides a helper to use the D-Bus secret service as credentials store.
+3. wincred: Provides a helper to use Windows credentials manager as store.
 
 ## Development