add README.md files to directories

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## This issue is a: (mark with an `x`)

- [ ] bug report
- [ ] request for new sample
- [X] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

## Further info:

The official Microsoft site documentation for Azure SDK for Go (docs.microsoft.com/go/azure) links to the following sample directories without a `README.md`:

* https://github.com/Azure-Samples/azure-sdk-for-go-samples/tree/master/compute
* https://github.com/Azure-Samples/azure-sdk-for-go-samples/tree/master/containerinstance
* https://github.com/Azure-Samples/azure-sdk-for-go-samples/tree/master/containerservice

There should be at least a short README in these directories so that users have more context after following links from the documentation site to the samples.

Review the compute, containerinstance, and containerservice sample directories in the repository, then compare their contents with the links from the Azure SDK for Go documentation. Add a short README.md to each directory that gives users context for the samples. Done means all three linked directories contain a README.md.

Written by the indexing model from the issue text.