Add docker2oci to implementations

docker2oci is a tool to convert a image saved from
`docker save` to oci format image.

It's kind of implementation of oci image format, it's a very
convenient tool to get a oci image. If you are interested,
I'd like add this to implementations.

```
$ docker save -o busybox.tar busybox
$ docker2oci -i busybox.tar busybox
```
or
```
$ docker save busybox | docker2oci busybox
```
Check the the image:
```
$ find busybox -type f
busybox/blobs/sha256/8baf43d43a34a0e6649c254b0200c2406fc40a501a852ba51a86ac3672dc0441
busybox/blobs/sha256/40a114053d955a2b80ee2cf6e13410b28b59594ceee9036b41e12c42d3e16615
busybox/blobs/sha256/8ac8bfaff55af948c796026ee867448c5b5b5d9dd3549f4006d9759b25d4a893
busybox/index.json
busybox/oci-layout
$ oci-image-tool validate busybox
oci-image-tool: reference "latest": OK
busybox: OK
Validation succeeded
$ mkdir bundle
$ oci-image-tool create --ref latest busybox bundle/
$ ls bundle/
config.json  rootfs

```

It also work if `docker save` save serveral images.
```
$ docker images busybox
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
busybox             v2                  0f4cf98c1107        4 minutes ago       1.09MB
busybox             v1                  df8dde2cf4ad        4 minutes ago       1.09MB
busybox             latest              2b8fd9751c4c        13 months ago       1.09MB
$ docker save busybox | docker2oci busybox
$ oci-image-tool validate busybox
oci-image-tool: reference "latest": OK
oci-image-tool: reference "v1": OK
oci-image-tool: reference "v2": OK
busybox: OK
Validation succeeded

```

Signed-off-by: Lei Jitang <[email protected]>

Author: coolljt0725

implementations.md

@@ -16,6 +16,6 @@ Projects or Companies currently adopting the OCI Image Specification
   - [containers/image](https://github.com/containers/image)
 * [coreos/rkt](https://github.com/coreos/rkt)
 * [box-builder/box](https://github.com/box-builder/box)
-
+* [coolljt0725/docker2oci](https://github.com/coolljt0725/docker2oci)
 
 _(to add your project please open a [pull-request](https://github.com/opencontainers/image-spec/pulls))_