docs: add a brief introduction in README (#83)

Resolves: #58
Signed-off-by: Lixia (Sylvia) Lei <lixlei@microsoft.com>

README.md

@@ -11,6 +11,10 @@
 <a href="https://oras.land/"><img src="https://oras.land/img/oras.svg" alt="banner" width="100px"></a>
 </p>
 
+`oras-credentials-go` is a credential management library designed for [`oras-go`](https://github.com/oras-project/oras-go). It supports reading, saving, and removing credentials from Docker configuration files and external credential stores that follow the [Docker credential helper protocol](https://docs.docker.com/engine/reference/commandline/login/#credential-helper-protocol).
+
+Once it reaches a fairly stable version (e.g. `v1.0.0-rc.1`), `oras-credentials-go` will be merged into `oras-go` (See [discussion](https://github.com/oras-project/oras-credentials-go/discussions/80)). After that, this repository will be archived.
+
 ## Versioning
 
 The `oras-credentials-go` library follows [Semantic Versioning](https://semver.org/), where breaking changes are reserved for MAJOR releases, and MINOR and PATCH releases must be 100% backwards compatible.