Add instructions on how to enable the docker bridge network (#352)

Signed-off-by: Andrew Roth <roth.andy@gmail.com>

docs/enable-docker-bridge-network.md

@@ -0,0 +1,23 @@
+# Enable Docker Bridge Network
+
+The latest versions of the AWS EKS-optimized AMI disable the docker bridge network by default. To enable it, add the `bootstrap_extra_args` parameter to your worker group template.
+
+```hcl
+locals {
+  worker_groups = [
+    {
+      # Other parameters omitted for brevity
+      bootstrap_extra_args = "--enable-docker-bridge true"
+    }
+  ]
+}
+```
+
+Examples of when this would be necessary are:
+
+- You are running Continuous Integration in K8s, and building docker images by either mounting the docker sock as a volume or using docker in docker. Without the bridge enabled, internal routing from the inner container can't reach the outside world.
+
+## See More
+
+- [Docker in Docker no longer works without docker0 bridge](https://github.com/awslabs/amazon-eks-ami/issues/183)
+- [Add enable-docker-bridge bootstrap argument](https://github.com/awslabs/amazon-eks-ami/pull/187)