Update tor-config README.md.

crates/tor-config/README.md

@@ -8,7 +8,47 @@ This crate is part of
 [Arti](https://gitlab.torproject.org/tpo/core/arti/), a project to
 implement [Tor](https://www.torproject.org/) in Rust.
 
-It provides low-level types for handling configuration values.
+It provides types for handling configuration values,
+and general machinery for configuration management.
+
+## Configuration in Arti
+
+The configuration for the `arti` command line program,
+and other programs which embed Arti reusing the configuration machinery,
+works as follows:
+
+ 1. We use [`tor_config::ConfigurationSources`](ConfigurationSources)
+    to enumerate the various places
+    where configuration information needs to come from,
+    and configure how they are to be read.
+    `arti` uses [`ConfigurationSources::from_cmdline`].
+
+ 2. [`ConfigurationSources::load`] actually *reads* all of these sources,
+    parses them (eg, as TOML files),
+    and returns a [`config::Config`].
+    This is a tree-structured dynamically typed data structure,
+    mirroring the input configuration structure, largely unvalidated,
+    and containing everything in the input config sources.
+
+ 3. We call one of the [`tor_config::resolve`](resolve) family.
+    This maps the input configuration data to concrete `ConfigBuilder `s
+    for the configuration consumers within the program.
+    (For `arti`, that's `TorClientConfigBuilder` and `ArtiBuilder`).
+    This mapping is done using the `Deserialize` implementations on the `Builder`s.
+    `resolve` then calls the `build()` method on each of these parts of the configuration
+    which applies defaults and validates the resulting configuation.
+
+    It is important to call `resolve` *once* for *all* the configuration consumers,
+    so that it sees a unified view of which config settings in the input
+    were unrecognized, and therefore may need to be reported to the user.
+    See the example in the [`load`] module documentation.
+
+ 4. The resulting configuration objects (eg, `TorClientConfig`, `ArtiConfig`)
+    are provided to the code that must use them (eg, to make a `TorClient`).
+
+See the
+[`tor_config::load` module-level documentation](load).
+for an example.
 
 ## ⚠ Stability Warning ⚠