docs: Update platform documentation

- Prefix platform documentation with 'platform-'
- Add/Correct links from top-level README.md
- Tweak the Azure documentation
- Move the vsudd README to platform-hyperkit.md
- Add a dummy document for qemu/kvm

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

docs/platform-gcp.md

@@ -0,0 +1,77 @@
+# Using LinuxKit on Google Cloud Platform (GCP)
+
+This is a quick guide to run LinuxKit on GCP. A lot of internal development and CI
+has used Google Cloud so the support is very good; other platforms will have similar support soon.
+
+## Setup
+
+You have two choices for authentication with Google Cloud
+
+1. You can use [Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials)
+2. You can use a Service Account
+
+### Application Default Credentials
+
+You need the [Google Cloud SDK](https://cloud.google.com/sdk/)
+installed.  Either install it from the URL or view `brew` (on a Mac):
+```shell
+brew tap caskroom/cask
+brew cask install google-cloud-sdk
+```
+
+Or via source code:
+
+```shell
+curl -SsL https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud-sdk-151.0.0-darwin-x86_64.tar.gz
+tar xzvf google-cloud-sdk-151.0.0-darwin-x86_64.tar.gz
+./google-cloud-sdk/install.sh
+```
+
+Then, set up some environment variables (adjust as needed) and login:
+```shell
+export CLOUDSDK_CORE_PROJECT=<GCP project>
+export CLOUDSDK_COMPUTE_ZONE=europe-west1-d
+gcloud auth login
+```
+
+The authentication will redirect to a browser with Google login.
+
+Also authenticate local applications with
+```
+gcloud auth application-default login
+```
+
+### Service Account
+
+You can use [this guide](https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances#createanewserviceaccount)
+to create a Service Account.
+
+Make sure to download the credentials in JSON format and store them somewhere safe.
+
+## Build an image
+
+When using `moby build ...` to build an image, specify `-output gcp-img` to
+build an image in a format that GCP will understand. For example:
+
+```
+moby build -output gcp-img myprefix.yml
+```
+
+This will create a local `myprefix.img.tar.gz` compressed image file.
+
+## Push image
+
+Do `linuxkit push gcp -project myproject-1234 -bucket bucketname myprefix.img.tar.gz` to upload it to the
+specified bucket, and create a bootable image from the stored image.
+
+Alternatively, you can set the project name and the bucket name using environment variables, `CLOUDSDK_CORE_PROJECT` and `CLOUDSDK_IMAGE_BUCKET`.
+See the constant values defined in [`src/cmd/linuxkit/run_gcp.go`](../src/cmd/linuxkit/run_gcp.go) for the complete list of the supported environment variables.
+
+## Create an instance and connect to it
+
+With the image created, we can now create an instance and connect to
+the serial port.
+
+```
+linuxkit run gcp -project myproject-1234 myprefix
+```