Updates Readme to include CFN launch command

During bug-bash it was realized that it would have been
helpful if there was cfn launch command available for launching
ECS updater. This change includes details in docs to fetch
Updater CFN template parameters and to launch it.

Author: srgothi92

README.md

@@ -20,6 +20,93 @@ When installed, the CloudFormation template will create the following resources
 * An IAM role for the Bottlerocket ECS Updater task itself as well as roles for Fargate and CloudWatch Events
 * SSM documents to query and execute updates on Bottlerocket instances
 
+## Getting Started
+
+To install the Bottlerocket ECS Updater, you will need to fetch some information first.
+
+### Subnet info
+
+You should either have a default virtual private cloud (VPC) or have already
+[created a VPC](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/get-set-up-for-amazon-ecs.html#create-a-vpc)
+in your account.
+
+To find your default VPC, run this command.
+(If you use an AWS region other than "us-west-2", make sure to change that.)
+
+```sh
+aws ec2 describe-vpcs \
+   --region us-west-2 \
+   --filters=Name=isDefault,Values=true \
+   | jq --raw-output '.Vpcs[].VpcId'
+```
+
+If you want to use a different VPC you created, run this to get the ID for your VPC.
+Make sure to change VPC_NAME to the name of the VPC you created.
+(If you use an EC2 region other than "us-west-2", make sure to change that too.)
+
+```sh
+aws ec2 describe-vpcs \
+   --region us-west-2 \
+   --filters=Name=tag:Name,Values=VPC_NAME \
+   | jq --raw-output '.Vpcs[].VpcId'
+```
+
+Next, run this to get information about the subnets in your VPC.
+It will give you a list of the subnets and tell you whether each is public or private.
+Make sure to change VPC_ID to the value you received from the previous command.
+(If you use an EC2 region other than "us-west-2", make sure to change that too.)
+
+```sh
+aws ec2 describe-subnets \
+   --region us-west-2 \
+   --filter=Name=vpc-id,Values=VPC_ID \
+   | jq '.Subnets[] | {id: .SubnetId, public: .MapPublicIpOnLaunch, az: .AvailabilityZone}'
+```
+
+You'll want to pick at least one and save it for the launch command later.
+Make sure the subnets you select have Internet access so the updater can reach its dependencies.
+Public subnets usually have Internet access via an [Internet gateway](https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Internet_Gateway.html) while private subnets may be configured with NAT.
+For more information, see [the VPC user guide](https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Internet_Gateway.html#vpc-igw-internet-access).
+
+We recommend picking several subnets in different availability zones.
+However, if you want to launch in a specific availability zone, make sure you pick a subnet that matches; the AZ is listed right below the public/private status.
+
+### Log Group
+
+You can either choose an existing log group or create a new one to get your ECS updater logs.
+
+You can run this to get the list of existing log-groups
+```sh
+aws logs describe-log-groups
+```
+
+You'll want to pick one and save it for the installation command later.
+
+If you want to create a new log group, run this (Make sure to provide LOG_GROUP_NAME)
+```sh
+aws logs create-log-group --log-group-name LOG_GROUP_NAME
+```
+
+### Install
+
+Now we can install the [CloudFormation template](stacks/bottlerocket-ecs-updater.yaml) to start the ECS updater for your cluster!
+
+There are a few values to make sure you change in this command:
+* CLUSTER_NAME: the name of the cluster you want ECS updater to manage Bottlerocket instances in
+* SUBNET_IDS: a comma-separated list of the subnets you selected earlier
+* LOG_GROUP_NAME: the log group name you selected or created earlier
+
+```sh
+aws cloudformation deploy \
+    --stack-name "bottlerocket-ecs-updater" \
+    --template-file "./stacks/bottlerocket-ecs-updater.yaml" \
+    --capabilities CAPABILITY_NAMED_IAM \
+    --parameter-overrides \
+    ClusterName="CLUSTER_NAME" \
+    Subnets="SUBNET_IDS" \
+    LogGroupName="LOG_GROUP_NAME"
+```
+
 ## How it works
 
 The Bottlerocket ECS Updater is designed to run as a scheduled Fargate task that queries, drains, and performs updates in your ECS cluster.