docs(interface): add docs about usage of django apps and interfaces

This commit adds some documentation about

* using the caluma django apps inside an existing django project
* supported interfaces.

Related to #940
Closes #941

Author: open-dynaMIX

README.md

@@ -19,6 +19,9 @@ Caluma Service is the core part of the Caluma project providing a
 
 ### Installation
 
+NOTE: We recommend using Caluma as a dedicated service. However, it is possible to integrate
+Caluma into a django project. You can read about this [here](docs/django-apps.md).
+
 **Requirements**
 * docker
 * docker-compose
@@ -74,5 +77,6 @@ For further information on our license choice, you can read up on the [correspon
 * [Validation](docs/validation.md) - Validation of user input
 * [Extending Caluma](docs/extending.md) - Extensions: Data visibility and
   permissions
+* [Using Caluma as django apps](docs/django-apps.md)
 * [Maintainer's Handbook](docs/maintainers.md) - HOWTO for various maintainer's
   tasks

docs/django-apps.md

@@ -0,0 +1,87 @@
+# Using Caluma as django apps
+
+## Disclaimer
+
+It is highly recommended to use Caluma as a dedicated service. However, there are usecases, where
+it might make sense to integrate Caluma into another django project.
+
+If you just want to get Caluma up and running, please see the documentation about [setting up
+the Caluma service](configuration.md).
+
+Please beware that Caluma only works with PostgreSQL, and requires the `psqlextra`
+backend to use the advanced features (such as JSON fields etc).
+
+## Installation
+
+Caluma is on PyPI, so you can just
+
+```shell
+pip install caluma
+```
+
+
+## Configuration
+
+### Installed apps
+
+Add the Caluma apps you want to use to your `INSTALLED_APPS`.
+
+Some notes about Caluma-internal dependencies:
+
+* `caluma_workflow` needs `caluma_form` to work correctly (as cases and work items point to documents)
+* `caluma_core` should always be installed when using Caluma (though, technically,
+  it would work without it, as core doesn't contain any models
+* `caluma_user` should always be added (same caveat as with core)
+
+```python
+INSTALLED_APPS = [
+    # ...
+    # Caluma and it's dependencies:
+    "caluma.caluma_core.apps.DefaultConfig",
+    "caluma.caluma_user.apps.DefaultConfig",
+    "caluma.caluma_form.apps.DefaultConfig",
+    "caluma.caluma_workflow.apps.DefaultConfig",
+    "caluma.caluma_data_source.apps.DefaultConfig",
+    "graphene_django",
+    "localized_fields",
+    "psqlextra",  # Caluma needs a PostgreSQL database using the psqlextra backend
+    "simple_history",
+    # ...
+]
+```
+
+### Settings
+
+Import the Caluma settings at the top of your `DJANGO_SETTINGS_MODULE`:
+
+```python
+from caluma.settings.caluma import *  # noqa
+```
+
+This will only load Caluma specific settings and no django specific ones (db, cache, etc.).
+
+Then you can configure caluma normally [using environment variables](configuration.md).
+
+
+### Database
+
+Caluma needs a PostgreSQL database using the `psqlextra` backend to use the advanced
+features (such as JSON fields etc).
+
+This is mandatory, that's why we tell you thrice.
+
+### Debug middleware
+
+It's recommended to add following lines to your settings file (after importing the caluma settings):
+
+```python
+# GraphQL
+if DEBUG:
+    GRAPHENE["MIDDLEWARE"].append("graphene_django.debug.DjangoDebugMiddleware")
+```
+
+
+## Supported interfaces
+
+See [interfaces](interfaces.md) for information about the officially supported API.
+