Add a first piece of explanation on the YAML config structure

docs/config-guide.md

@@ -1,3 +1,93 @@
 # A VPP Configuration Utility
 
 ## Configuration Guide
+
+`vppcfg` consumes YAML files of a specific format. Their validity is asserted by two main
+types of validation: 
+
+1.  syntax checks are performed by [Yamale](https://github.com/23andMe/Yamale/) and this
+    ensures that all fields in the YAML file are correctly formed, that field-names are
+    correctly spelled, that no extra fields are given, and their values are of the correct
+    type.
+1.  semantic validations are performed to ensure that configurations are safely applyable
+    to a running VPP. *Note*: Some semantic checks are stricter than VPP, because applying
+    them may leave the dataplane in a non-recoverable state.
+
+For the curious, the Yamale syntax validation lives in [this schema](../schema.yaml).
+
+### Basic structure
+
+The YAML configuration file has the following structure, consisting of several _maps_ of
+a given object _type_, which specify _names_ of those objects:
+
+```
+loopbacks:
+  loop0:
+    [ Loopback Configuration ]
+
+bondethernets:
+  BondEthernet0:
+    [ BondEthernet (bond) Configuration ]
+
+vxlan_tunnels:
+  vxlan_tunnel0:
+    [ VXLAN (tunnel) Configuration ]
+
+bridgedomains:
+  bd1:
+    [ BridgeDomain Configuration ]
+
+interfaces:
+  GigabitEthernet3/0/0:
+    [ Interface Configuration ]
+  BondEthernet0:
+    [ BondEthernet (interface) Configuration ]
+  vxlan_tunnel0:
+    [ VXLAN (interface) Configuration ]
+```
+
+Object _names_ are strictly enforced, they must be unique in their scope, and they are case sensitive.
+For example, any loopback MUST be named `loopN`, and any bondethernet MUST be named `BondEthernetN`
+(note here the camel case). A distinction is made between the object and the resulting interface:
+A BondEthernet occurs twice in the configuration. The first time, in the `bondethernets` section, the
+bond configuration is specified. That bond configuration yields an interface in VPP named BondEthernetN,
+which can then be manipulated as any other interface (eg. have IP addresses, Linux Control Plane
+names, sub-interfaces and so on). The same is true for VXLAN tunnels, the only currently supported
+tunnel type.
+
+### Loopbacks
+
+Loopbacks are required to be named `loopN` where N is [0,4096). The configuration contains the
+following fields:
+
+*   ***description***: A string, no longer than 64 characters, and excluding the single quote '
+    and double quote ". This string is currently not used anywhere, and serves for enduser
+    documentation purposes.
+*   ***lcp***: A Linux Control Plane interface pair _LIP_. If specified, the loopback will be
+    presented in Linux under this name. Its name may be at most 15 characters long, and match
+    the regular expression `[a-z]+[a-z0-9-]*`.
+*   ***mtu***: An integer value between [128,9216], noting the (packet) MTU of the loopback. It
+    will default to 1500 if not specified.
+*   ***addresses***: A list of between one and six IPv4 or IPv6 addresses including prefixlen
+    in CIDR format. VPP requires IP addresses to be unique in the entire dataplane, with one
+    notable exception: Multiple IP addresses in the same prefix/len can be added on one and the
+    same interface.
+
+Examples:
+```
+loopbacks:
+  loop0:
+    lcp: "lo0"
+    addresses: [ 10.0.0.1/32, 2001:db8::1/128 ]
+  loop1:
+    lcp: "bvi1"
+    addresses: [ 10.0.1.1/24, 10.0.1.2/24, 2001:db8:1::1/64 ]
+```
+
+### Bridge Domains
+
+### BondEthernets
+
+### Interfaces
+
+### VXLAN Tunnels