Add doc for using custom CA

docs/custom_ca.md

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+# Using a custom certificate authority
+
+## Background Info
+
+When deploying a `kops` based Kubernetes cluster, `kops` will generate a certificate authority keypair for signing
+various certificates with. In some cases, you may want to provide your own CA keypair.
+
+Another use case would be to use the CA keypair of another cluster if you are creating many
+short lived clusters and don't want to create a unique CA for each one.
+
+### Building a cluster with a custom CA
+
+The following procedure will allow you to override the CA when creating a cluster. For the sake of this example, you have two files
+`ca.crt` and `ca.key`. 
+
+>`cluster-name.com` should be the cluster name you put in the `cluster.yaml`
+
+```bash
+kops create -f cluster.yaml
+kops create secret keypair ca --cert ca.crt --key ca.key --name cluster-name.com
+kops update cluster --yes
+```
+
+1. First we create the cluster folder structure in the statestore.
+2. Second, we create a `Secret` of type `Keypair` with the name `ca` and provide our own values.
+3. Lastly, we run `kops update cluster --yes`, which will generate all the certificates needed, referencing the `Secret` called `ca` we just defined (versus generating its own).
+
+### Using a previous `kops` cluster CA
+
+In some cases you will want to create a cluster and use the CA generated in a previous `kops` cluster.
+To do so, you will need to copy the CA files from the state store, and then use them as values in the above procedure.
+
+The files are located as follows:
+
+`s3://state-store/<cluster-name>/pki/issued/ca/<id>.crt`
+
+`s3://state-store/<cluster-name>/pki/private/ca/<id>.key`