feat(meta): Add README

meta/README.md

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+Metadata of the DGNum infrastructure
+====================================
+
+# DNS
+
+The DNS configuration of our infrastructure is completely defined with the metadata contained in this folder.
+
+The different machines have records pointing to their IP addresses when they exist:
+
+- $node.$site.infra.dgnum.eu points IN A $ipv4
+- $node.$site.infra.dgnum.eu points IN AAAA $ipv6
+
+- v4.$node.$site.infra.dgnum.eu points IN A $ipv4
+- v6.$node.$site.infra.dgnum.eu points IN AAAA $ipv6
+
+Then the services hosted on those machines can be accessed through redirections:
+
+- $service.dgnum.eu IN CNAME $node.$site.infra.dgnum.eu
+
+or, when targeting only a specific IP protocol:
+
+- $service4.dgnum.eu IN CNAME ipv4.$node.$site.infra.dgnum.eu
+- $service6.dgnum.eu IN CNAME ipv6.$node.$site.infra.dgnum.eu
+
+Extra records exist for ns, mail configuration, or the main website but shouldn't change or be tinkered with.
+
+# Network
+
+The network configuration (except the NetBird vpn) is defined statically.
+
+TODO.
+
+# Nixpkgs
+
+Machines can use different versions of NixOS, the supported and default ones are specified here.
+
+# Nodes
+
+The nodes are declared statically, several options can be configured:
+
+- `deployment`, the colmena deployment option
+- `stateVersion`, the state version of the node
+- `nixpkgs`, the version of NixOS to use
+- `admins`, the list of administrators specific to this node, they will be given root access
+- `adminGroups`, a list of groups whose members will be added to `admins`
+- `site`, the physical location of the node
+- `vm-cluster`, the VM cluster hosting the node when appropriate
+
+Some options are set automatically, for example:
+
+- `deployment.targetHost` will be inferred from the network configuration
+- `deployment.tags` will contain `infra-$site`, so that a full site can be redeployed at once
+
+# Organization
+
+The organization defines the groups and members of the infrastructure team,
+one day this information will be synchronized in Kanidm.
+
+## Members
+
+For a member to be allowed access to a node, they must be defined in the `members` attribute set,
+and their SSH keys must be available in the keys folder.
+
+## Groups
+
+Groups exist only to simplify the management of accesses:
+
+- The `root` group will be given administrator access on all nodes
+- The `iso` group will have its keys included in the ISOs built from the iso folder
+
+Extra groups can be created at will, to be used in node-specific modules.
+
+# Module
+
+The meta configuration can be evaluated as a module, to perform checks on the structure of the data.