docs: no mention is made for snapshotters on windows

### Description

For the documentation at [`docs/snapshotters/README.md`](https://github.com/containerd/containerd/blob/32169d591dbc6133ef7411329b29d0c0433f8c4d/docs/snapshotters/README.md), no mention is made for Windows platform snapshotters. Linux is assumed right from the start.

Will be nice to add some notes about these snapshotters.

```bash
PS> ctr plugins ls
TYPE ID PLATFORMS STATUS
io.containerd.snapshotter.v1 windows-lcow linux/amd64 ok
io.containerd.snapshotter.v1 windows windows/amd64 ok
```

### Steps to reproduce the issue

Issue with the docs.

### Describe the results you received and expected

N/A

### What version of containerd are you using?

1.7.13

### Any other relevant information

_No response_

### Show configuration if it is related to CRI plugin.

_No response_