add sample for creating disk snapshots

To create a snapshot use the [SnapshotsClient](https://godoc.org/github.com/Azure/azure-sdk-for-go/services/compute/mgmt/2018-06-01/compute#SnapshotsClient), specifically the [CreateOrUpdate](https://godoc.org/github.com/Azure/azure-sdk-for-go/services/compute/mgmt/2018-06-01/compute#SnapshotsClient.CreateOrUpdate) method. That method takes a [Snapshot](https://godoc.org/github.com/Azure/azure-sdk-for-go/services/compute/mgmt/2018-06-01/compute#Snapshot) object, whose most significant property is [DiskProperties](https://godoc.org/github.com/Azure/azure-sdk-for-go/services/compute/mgmt/2018-06-01/compute#DiskProperties).

The Go binding maps to the [compute/snapshots/createorupdate](https://docs.microsoft.com/en-us/rest/api/compute/snapshots/createorupdate) API.

Start with the linked SnapshotsClient.CreateOrUpdate method and the Snapshot and DiskProperties types in the Azure compute Go binding. Add a sample showing disk snapshot creation through the compute/snapshots/createorupdate API; it is done when the repository contains a working example of that operation.

Written by the indexing model from the issue text.