Add initial CI subsystem documentation

ci/README.md

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+
+Our CI infrastructure is built around [BuildKite](https://buildkite.com) with some
+additional GitHub integration provided by https://github.com/mvines/ci-gate
+
+## Buildkite AWS CloudFormation Setup
+
+We use AWS CloudFormation to scale machines up and down based on the current CI
+load.  If no machine is currently running it can take up to 60 seconds to spin
+up a new instance, please remain calm during this time.
+
+### Agent Queues
+
+We define two [Agent Queues](https://buildkite.com/docs/agent/v3/queues):
+`queue=default` and `queue=cuda`.  The `default` queue should be favored and
+runs on lower-cost CPU instances.  The `cuda` queue is only necessary for
+running **tests** that depend on GPU (via CUDA) access -- CUDA builds may still
+be run on the `default` queue, and the [buildkite artifact
+system](https://buildkite.com/docs/builds/artifacts) used to transfer build
+products over to a GPU instance for testing.
+
+### AMI
+We use a custom AWS AMI built via https://github.com/solana-labs/elastic-ci-stack-for-aws/tree/solana/cuda.
+
+Use the following process to update this AMI as dependencies change:
+```bash
+$ export AWS_ACCESS_KEY_ID=my_access_key
+$ export AWS_SECRET_ACCESS_KEY=my_secret_access_key
+$ git clone https://github.com/solana-labs/elastic-ci-stack-for-aws.git -b solana/cuda
+$ cd elastic-ci-stack-for-aws/
+$ make build
+$ make build-ami
+```
+
+Watch for the *"amazon-ebs: AMI:"* log message to extract the name of the new
+AMI.  For example:
+```
+amazon-ebs: AMI: ami-07118545e8b4ce6dc
+```
+The new AMI should also now be visible in your EC2 Dashboard.  Go to the desired
+AWS CloudFormation stack, update the **ImageId** field to the new AMI id, and
+*apply* the stack changes.
+
+