Add brief source code reference documentation.

NEWS

@@ -2,6 +2,8 @@
  * #727: Add a MariaDB database hook that uses native MariaDB commands instead of the deprecated
    MySQL ones. Be aware though that any existing backups made with the "mysql_databases:" hook are
    only restorable with a "mysql_databases:" configuration.
+ * Add a source code reference for getting oriented with the borgmatic code as a developer:
+   https://torsion.org/borgmatic/docs/reference/source-code/
 
 1.8.1
  * #326: Add documentation for restoring a database to an alternate host:

docs/how-to/develop-on-borgmatic.md

@@ -37,6 +37,9 @@ install (so without `--user`, as root), or even into a
 borgmatic is up to you, but generally an editable install makes development
 and testing easier.
 
+To get oriented with the borgmatic source code, have a look at the [source
+code reference](https://torsion.org/borgmatic/docs/reference/source-code/).
+
 
 ## Automated tests
 

docs/reference/source-code.md

@@ -0,0 +1,29 @@
+---
+title: Source code reference
+eleventyNavigation:
+  key: 🐍 Source code reference
+  parent: Reference guides
+  order: 3
+---
+## getting oriented
+
+If case you're interested in [developing on
+borgmatic](https://torsion.org/borgmatic/docs/how-to/develop-on-borgmatic/),
+here's an abridged primer on how its Python source code is organized to help
+you get started. At the top level we have:
+
+ * [borgmatic](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/borgmatic): The main borgmatic source module. Most of the code is here.
+ * [docs](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/docs): How-to and reference documentation, including the document you're reading now.
+ * [sample](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/sample): Example configurations for cron and systemd.
+ * [scripts](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/scripts): Dev-facing scripts for things like building documentation and running end-to-end tests.
+ * [tests](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/tests): Automated tests organized by: end-to-end, integration, and unit.
+
+Within the `borgmatic` directory you'll find:
+
+ * [actions](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/borgmatic/actions): Mid-level code for running each borgmatic action (create, list, check, etc.).
+ * [borg](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/borgmatic/borg): Lower-level code that actually shells out to Borg for each action.
+ * [commands](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/borgmatic/commands): Looking to add a new flag or action? Start here. This contains borgmatic's entry point, argument parsing, and shell completion. 
+ * [config](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/borgmatic/config): Code responsible for loading, normalizing, and validating borgmatic's configuration.
+ * [hooks](https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/borgmatic/hooks): Looking to add a new database or monitoring integration? Start here.
+
+So, broadly speaking, the control flow goes: `commands` → `config` followed by `commands` → `actions` → `borg` and `hooks`.