Update godoc badge (#139)

Signed-off-by: Shiwei Zhang <shizh@microsoft.com>

README.md

@@ -13,6 +13,8 @@ The ORAS Go library follows [Semantic Versioning](https://semver.org/), where br
 
 ### v1: stable
 
+[![GoDoc](https://godoc.org/github.com/oras.land?status.svg)](https://godoc.org/oras.land/oras-go)
+
 As there are various stable projects depending on the ORAS Go library, the
 [`v1`](https://github.com/oras-project/oras-go/tree/v1) branch
 is maintained for API stability, dependency updates, and security patches.
@@ -23,6 +25,8 @@ to use releases with major version `1`.
 
 ### v2: experimental
 
+[![GoDoc](https://godoc.org/github.com/oras.land?status.svg)](https://godoc.org/oras.land/oras-go/v2)
+
 In contrast to the `v1` branch, the
 [`main`](https://github.com/oras-project/oras-go/tree/main) branch
 is where all new feature development will occur. Since ORAS is a