chore/release: deprecation notice for pure docker (#1358)

closes PLAT-416

This just adds a deprecation notice for pure docker

### Checklist

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* [ ] Sister
[deploy-sourcegraph](https://github.com/sourcegraph/deploy-sourcegraph)
change:
* [ ] Sister
[customer-replica](https://github.com/sourcegraph/deploy-sourcegraph-docker-customer-replica-1)
change (if necessary, for any changes affecting pure-docker or
configuration):
* [ ] All images have a valid tag and SHA256 sum
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Author: DaedalusG

README.md

@@ -31,6 +31,8 @@ We've made our deployment configurations open source to better serve our custome
 
 ## Pure-Docker Sourcegraph cluster deployment reference
 
+> ⚠️ **Deprecation Notice:** The pure-Docker deployment method is planned for deprecation.
+
 What if your organization wants a multi-machine deployment without using Kubernetes?
 What if you use a different container management platform, for example?
 Anyone using a container management platform other than Kubernetes (Netflix's [Titus](https://netflix.github.io/titus/), Apache's [Mesos](http://mesos.apache.org/documentation/latest/docker-containerizer/), etc.) can use our [Pure-Docker Sourcegraph cluster deployment reference](./pure-docker/README.md) to deploy Sourcegraph.

pure-docker/README.md

@@ -1,5 +1,7 @@
 # Pure-Docker Sourcegraph cluster deployment reference
 
+> ⚠️ **Deprecation Notice:** The pure-Docker deployment method is planned for deprecation.
+
 ## Deploying
 
 First clone the repository, then: