aws/publicIpv4Pool/doc: basic user docs

User documentation for the Public IPv4 Pool feature (BYOIPv4)
on install-config, where the customer can specify the Public IPv4 Pool
ID from a Public IPv4 CIDR pool that had bought to the AWS account.

docs/user/aws/custom_public_ipv4.md

@@ -0,0 +1,38 @@
+# Install OpenShift on AWS using custom/owned Public IPv4 Pool
+
+Steps to create a cluster on AWS using Public IPv4 address pool
+that you bring to your AWS account with BYOIP.
+
+## Prerequisites
+
+- Public IPv4 Pool Provisioned in the Account
+- Total of ( (Zones*3 ) + 1) of Public IPv4 available in the pool, where: Zones is the total numbber of AWS zones used to deploy the OpenShift cluster.
+    - Example to query the IPv4 pools available in the account, which returns the  `TotalAvailableAddressCount`:
+```
+aws ec2 describe-public-ipv4-pools --region us-east-1
+```
+
+## Steps
+
+- Create the install config setting the field `platform.aws.publicIpv4PoolId`, and create the cluster:
+
+```yaml
+apiVersion: v1
+baseDomain: ${CLUSTER_BASE_DOMAIN}
+metadata:
+  name: ocp-byoip
+platform:
+  aws:
+    region: ${REGION}
+    publicIpv4Pool: ipv4pool-ec2-123456789abcde
+publish: External
+pullSecret: '...'
+sshKey: |
+  '...'
+```
+
+- Create the cluster
+
+```sh
+openshift-install create cluster
+```