CONTRIBUTING: add Development section

Intends to do two things:
* Give new contributors a high level view of the
  source code and architecture.
* Document our agreed but not written coding conventions.

Signed-off-by: Attila Szakacs <[email protected]>

CONTRIBUTING.md

@@ -12,8 +12,12 @@ easier.
     1. [Reporting bugs](#reporting-bugs)
     2. [Feature requests](#feature-requests)
     3. [Testing](#testing)
-2. [Pull requests](#pull-requests)
-3. [Additional resources](#additional-resources)
+2. [Development](#development)
+    1. [Source code structure](#source-code-structure)
+    2. [Architectural overview](#architectural-overview)
+    3. [Coding guidelines](#coding-guidelines)
+3. [Pull requests](#pull-requests)
+4. [Additional resources](#additional-resources)
 
 ## Issues
 
@@ -56,6 +60,77 @@ requests - there's only so much [automated testing][ar:github-actions] can do.
 For example, you can help testing on platforms the developers do not
 have access to, or try configurations not thought of before.
 
+## Development
+
+### Source code structure
+
+The code is organized into:
+- Core library (`lib` directory).
+- Plugin drivers (`modules` directory).
+
+The configuration language is defined in `lib/cfg-grammar.y` and module related `...-grammar.ym` files.
+
+Supported build systems are Autotools and CMake.
+
+Testing:
+- Unit tests use the [Criterion][ar:criterion] framework, and can be found alongside its respective source code (`.../tests` directory).
+- End-to-end tests are written in Python with `pytest` (`tests/light` directory).
+
+Packaging:
+- Located in the `packaging` directory, including Alpine-based container images, Helm charts, and Debian/RPM packages.
+
+### Architectural overview
+
+#### Main event loop
+
+AxoSyslog uses [`ivykis`][ar:ivykis] for the main event loop.
+
+#### Log processing flow
+
+Logs are received from local/remote `source`s, processed with transformations (`parser`s, `rewrite`s, `filter`s, or `filterx`), and routed to `destination`s.
+
+`source`s can be:
+- Non-blocking, socket-based (see `lib/logreader`).
+- Blocking, threaded (see `lib/logthrsource`).
+
+`destination`s can be:
+- Non-blocking, socket-based (see `lib/logwriter`).
+- Blocking, threaded (see `lib/logthrdest`).
+
+### Coding guidelines
+
+AxoSyslog is mainly written in C, with an object-oriented approach. Some modules are implemented in C++, Python or Java.
+
+AxoSyslog follows clean coding principles:
+- Functionality is layered in abstractions to enhance readability and maintainability.
+- Function and variable names are descriptive to accurately communicate each layer's purpose.
+
+#### C code format
+
+AxoSyslog follows GNU standards with additional conventions:
+- Classes are in PascalCase, functions and variables in snake_case.
+- Class namespaces are prefixed in function names.
+- Private methods are `static` and start with an underscore (`_`).
+- Function signatures are split to two lines (return type on the first, rest on the second) to help `git grep ^my_func_impl` searches.
+- See [Patches](#patches).
+
+#### Git history
+
+- Each commit is atomic and detailed in the commit message.
+- Commits represent the logical buildup of a feature, organized via interactive rebase to improve readability and maintainability.
+- See [Commits](#commits).
+
+#### Comments
+
+- Comments are rarely used, as clean coding and atomic commits are the main way to document the code.
+- Use comments where architectural overviews are needed or when interface contracts cannot be enforced.
+
+#### Code quality
+
+AxoSyslog thrives for the highest quality, but supports developing features incrementally:
+- Core abstractions or widely dependent code must be clean, optimized, and well-tested.
+- Self-contained features may have relaxed standards but are iterated upon for improvement.
+
 ## Pull requests
 
 If you plan to open a pull request, please follow the guideline below.
@@ -205,3 +280,5 @@ testing.
  [ar:issue-tracker]: https://github.com/axoflow/axosyslog/issues
  [ar:issues:good-first-issue]: https://github.com/axoflow/axosyslog/labels/good%20first%20issue
  [ar:github-actions]: https://github.com/axoflow/axosyslog/actions
+ [ar:criterion]: https://github.com/Snaipe/Criterion
+ [ar:ivykis]: https://github.com/buytenh/ivykis