add network configuration instructions to documentation

[bxy4543]

Some machines do not support the use of the default calico configuration, Before creating this manifest, read its contents and make sure its settings are correct for your environment. For example, you may need to change the default IP pool CIDR to match your pod network CIDR. the network configuration related issues need to be described separately in the document.

calico docs
Before you begin
Make sure you have a linux host that meets the following requirements:
x86-64, arm64, ppc64le, or s390x processor
2CPU
2GB RAM
10GB free disk space
RedHat Enterprise Linux 7.x+, CentOS 7.x+, Ubuntu 16.04+, or Debian 9.x+
Ensure that Calico can manage cali and tunl interfaces on the host. If NetworkManager is present on the host, refer to Configure NetworkManager.

[allencloud]

@bxy4543 Before talking about the calico configuration topic, I wish to make it clear that WHAT IS INCLUDED IN SEALER.

From my initial opinion, base image of sealer should not be included in sealer's scope. For what could be done from sealer for base CloudImage, here is my suggested categories:

• several sample base cloudimages, but cannot provide every kinds of base cloudimage end users wish to have. sealer is an open-source tool, it cannot be a commercial organization to provide sufficient base cloudimage. Although sealer community currently provides some, we should separate that from the sealer project's own goal.
• provide the base CloudImage building way for end-users. sealer could not provide everything, but sealer could provide the way for end-users to create things by themselves.
• add documents to clarify explicitly what is the exact relationship between base CloudImage and sealer binary. This is the technical part our sealer project should pay more emphasis on. And there must be some protocol sealer and base CloudImage should obey to run a distributed application in CloudImage.