Contributing guide (#109)

* Begin work on a contributing guide

* Small updates and link in README

CONTRIBUTING.md

@@ -0,0 +1,84 @@
+# Contributing guide
+
+This document describes the organization of the neural-fortran codebase to help
+guide the code contributors.
+
+## Overall code organization
+
+The source code organization follows the usual `fpm` convention:
+the library code is in [src/](src/), test programs are in [test/](test/),
+and example programs are in [example/](example/).
+
+The top-level module that suggests the public, user-facing API is in
+[src/nf.f90](src/nf.f90).
+All other library source files are in [src/nf/](src/nf/).
+
+Most of the library code defines interfaces in modules and implementations in
+submodules.
+If you want to know only about interfaces, in other words how to call procedures
+and what these procedures return, you can read just the module source files and
+not worry about the implementation.
+Then, if you want to know more about the implementation, you can find it in the
+appropriate source file that defines the submodule.
+Each library source file contains either one module or one submodule.
+The source files that define the submodule end with `_submodule.f90`.
+
+## Components
+
+Neural-fortran defines several components, described in a roughly top-down order:
+
+* Networks
+* Layers
+  - Layer constructor functions
+  - Concrete layer implementations
+* Optimizers
+* Activation functions
+
+### Networks
+
+A network is the main component that the user works with,
+and the highest-level container in neural-fortran.
+A network is a collection of layers.
+
+The network container is defined by the `network` derived type
+in the `nf_network` module, in the [nf_network.f90](src/nf/nf_network.f90)
+source file.
+
+In a nutshell, the `network` type defines an allocatable array of `type(layer)`
+instances, and several type-bound methods for training and inference.
+
+### Layers
+
+Layers are the main building blocks of neural-fortran and neural networks in
+general.
+There is a common, high-level layer type that maintains the data flow
+in and out and calls the specific layer implementations of forward and backward
+pass methods.
+
+When introducing a new layer type, study how the [dense](src/nf/nf_dense_layer.f90)
+or [convolutional](src/nf/nf_conv2d_layer.f90) concrete types are defined and
+implemented in their respective submodules.
+You will also need to follow the same use pattern in the
+[high-level layer type](src/nf/nf_layer.f90) and its corresponding submodule.
+
+### Optimizers
+
+Optimizers are the algorithms that determine how the model parameters are
+updated during training.
+
+Optimizers are currently implmented in the [nf_optimizers.f90](src/nf/nf_optimizers.f90)
+source file and corresponding module.
+An optimizer instance is passed to the network at the `network % train()` call
+site.
+
+### Activation functions
+
+Activation functions and their derivatives are defined in the
+[nf_activation.f90](src/nf/nf_activation.f90) source file and corresponding
+types.
+They are implemented using a base activation abstract type and concrete types
+for each activation function.
+When implementing a new activation function in the library, you need to define
+a new concrete type that extends the abstract activation function type.
+The concrete type must have `eval` and `eval_prime` methods that evaluate the
+function and its derivative, respectively.

README.md

@@ -11,6 +11,7 @@ Read the paper [here](https://arxiv.org/abs/1902.06714).
   - [Building with CMake](https://github.com/modern-fortran/neural-fortran#building-with-cmake)
 * [Examples](https://github.com/modern-fortran/neural-fortran#examples)
 * [API documentation](https://github.com/modern-fortran/neural-fortran#api-documentation)
+* [Contributing](https://github.com/modern-fortran/neural-fortran#contributing)
 * [Acknowledgement](https://github.com/modern-fortran/neural-fortran#acknowledgement)
 * [Related projects](https://github.com/modern-fortran/neural-fortran#related-projects)
 
@@ -235,6 +236,11 @@ ford ford.md
 from the neural-fortran top-level directory to generate the API documentation in doc/html.
 Point your browser to doc/html/index.html to read it.
 
+## Contributing
+
+This [Contributing guide](CONTRIBUTING.md) briefly describes the code organization.
+It may be useful to read if you want to contribute a new feature to neural-fortran.
+
 ## Acknowledgement
 
 Thanks to all open-source contributors to neural-fortran: