contrib: add first draft of agent configuration docs

contributing/config/README.md

@@ -1,3 +1,58 @@
 # Agent Configuration
 
-- [Checklist for adding a new field](./checklist-adding-config-fields.md)
+The [Agent Configuration] is the primary mechanism for configuring Consul. Agent
+Configuration also allows for specifying [Config Entries], [Services], and [Checks] that
+will be loaded when the agent starts.
+
+Most configuration comes from [hcl] or `json` files, but some configuration can also be
+specified using command line flags, and some can be loaded with [Auto-Config].
+
+See also the [checklist for adding a new field] to the configuration.
+
+[hcl]: https://github.com/hashicorp/hcl/tree/hcl1
+[Agent Configuration]: https://www.consul.io/docs/agent/options
+[checklist for adding a new field]: ./checklist-adding-config-fields.md
+[Auto-Config]: #auto-config
+[Config Entries]: https://www.consul.io/docs/agent/options#config_entries
+[Services]: https://www.consul.io/docs/discovery/services
+[Checks]: https://www.consul.io/docs/discovery/checks
+
+
+## Code
+
+The Agent Configuration is implemented in [agent/config], and the primary entrypoint is
+[Load]. Config loading is performed in phases:
+
+1. Command line flags are used to create a `config.LoadOpts` and passed to `Load`.
+2. `Load` reads all the config files and builds an ordered list of `config.Source`.
+3. Each `config.Source` is read to produce a `config.Config`.
+4. Each `config.Config` is merged ontop the previous.
+5. A `config.RuntimeConfig` is produced from the merged `config.Config`
+6. The `config.RuntimeConfig` is validated.
+7. Finally a result is returned with the `RuntimeConfig` and any warnings, or an error.
+
+[agent/config]: https://github.com/hashicorp/consul/tree/main/agent/config
+[Load]: https://pkg.go.dev/github.com/hashicorp/consul/agent/config#Load
+
+If [Auto-Config] is enabled, when it receives the config from the server, the
+entire process is repeated a second time with the addition config provided as another
+`config.Source`.
+
+Default values can be specified in one of the [default sources] or set when
+converting from `Config` to `RuntimeConfig` in [builder.build]. Hopefully in the future we
+should remove one of those ways of setting default values.
+
+[default sources]: https://github.com/hashicorp/consul/blob/main/agent/config/default.go
+[builder.build]: https://github.com/hashicorp/consul/blob/main/agent/config/builder.go
+
+## Auto-Config
+
+Auto-Config is enabled by the [auto_config] field in an Agent Configuration file. It is
+implemented in a couple packages.
+
+* the server RPC endpoint is in [agent/consul/auto_config_endpoint.go]
+* the client that receives and applies the config is implemented in [agent/auto-config]
+
+[auto_config]: https://www.consul.io/docs/agent/options#auto_config
+[agent/consul/auto_config_endpoint.go]: https://github.com/hashicorp/consul/blob/main/agent/consul/auto_config_endpoint.go
+[agent/auto-config]: https://github.com/hashicorp/consul/tree/main/agent/auto-config