Document how to use Composer in development.

docs/development/composer.md

@@ -0,0 +1,34 @@
+# Composer
+
+The farmOS development Docker image comes pre-installed with
+[Composer](https://getcomposer.org), which is used for dependency management.
+
+## Running Composer in Docker
+
+In order to run the `composer` command, you must use `docker exec` to run the
+command inside the farmOS container.
+
+    sudo docker exec -it -e COMPOSER_MEMORY_LIMIT=-1 farmos_www_1 composer
+
+For example, the following will run the `composer update` command:
+
+    sudo docker exec -it -e COMPOSER_MEMORY_LIMIT=-1 farmos_www_1 composer update
+
+## Common tasks
+
+Some common Composer tasks are documented here.
+
+### Updating dependencies
+
+    composer update
+
+### Adding a module
+
+    composer require drupal/[module]
+
+This will download the module into the `web/modules/contrib` directory, and add
+it to the root `composer.json` file.
+
+If the module is being added to the farmOS installation profile itself, you
+need to manually move the `require` line from the root `composer.json` to
+`web/profiles/farm/composer.json` and commit it to that repository.