Restore Plugin Documentation (#148)

Author: gigincg

docs/care/CEP/Completed /0005-plugin-manager.md

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+
+# CEP-5: Care Apps
+
+## Requirements/Motive 
+
+Since Care is being used in different environments for different use-cases, there must exist some method to efficiently create modifications which does not
+impact unrelated deployments.
+
+Some type of plugin system is required to achieve this.
+
+Plugins can be abstracted standalone features, they should be capable of being maintained as separate packages, this allows easy 3rd party integrations as well.
+
+Care must be modified to allow plugins to be easily created and configured.
+
+## Implementation
+
+To solve the problem in an abstracted way, a new package manager would be added onto care's core system. The core system
+will be responsible for maintaining the bare necessary components to operate a facility ( or a hospital ). Everything other
+than that can be made into apps that can be composed together to form deployments as required. 
+
+A plugin will be called as a `plug` or an `app` ( `app` is already defined by django, so `plug` is used in those cases to avoid confusion ) 
+
+Plugs will be managed in care through a plug manager and configured via a plug config, the plug config would take care of composing and defining plugs.  
+
+Plugs will be added at build time rather than runtime, this allows care to be deployed in areas with no internet connectivity. At build time
+the plug manager would iterate through each plug and install it along with its requirements. plugs will also have hooks to be executed at this point.
+
+Plugs should not directly edit database structures maintained by the core system, plugs can have their own models and maintain relations if needed. 
+
+Django signals can be used to listen for changes in the core system and react to them as needed. Custom signals can be created in the core system as needed 
+to cater to the needs of the plugs.
+
+## Plug Config
+
+The plug config is located at the root of the project in the `plug_config.py` file.  
+
+```python
+
+from plugs.manager import PlugManager
+from plugs.plug import Plug
+
+my_plugin = Plug(
+    name="my_plugin",
+    package_name="git+https://github.com/octo/my_plugin.git",
+    version="@v1.0.0",
+    configs={
+        "SERVICE_API_KEY": "my_api_key",
+        "SERVICE_SECRET_KEY": "my_secret_key",
+        "VALUE_1_MAX": 10,
+    },
+)
+
+plugs = [my_plugin]
+
+manager = PlugManager(plugs)
+```
+
+The plugs variable is defined as a list of plugs, the developer can add plugins as needed here to compose a deployment. The configs dict 
+can be used to override variables that the plug uses, this can also hold functions if needed to get fine grain control over the deployment.
+
+
+References: 
+- https://github.com/openedx/edx-django-utils/tree/master/edx_django_utils/plugins
+- https://github.com/useblocks/groundwork