Update readme with prerequisite for existing vcn

[pvaldria]

For existing VCN - Check Security list have ports open.
Have NAT Gateway configured for private subnets to access internet for yum installs to work
Ensure route rules exist for private subnet to use NAT gateway
Ensure route rules exist for public subnet to use Internet gateway
Public subnet and Private Subnets have SL for egress for 0.0.0.0/0
Check what SL is needed for ingress on Public and Private subnet.