Added CONTRIBUTING file

CONTRIBUTING.md

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+Thanks for considering contributing your work to make Platypush a better product!
+
+Contributions are more than welcome, and the follow the standard Gitlab procedure:
+
+- [Fork the repo](https://git.platypush.tech/platypush/platypush).
+- Prepare your changes.
+- [Submit a merge request](https://git.platypush.tech/platypush/platypush/-/merge_requests).
+
+Guidelines:
+
+- The code should ideally have no LINT warnings/issues.
+
+- Project conventions:
+  - 4 spaces to indent
+  - RST format for classes and methods documentation 
+  - Run `generate_missing_docs.sh` if you are adding new plugins/backends to automatically
+    generate the doc templates
+  - Naming conventions: plugin classes are named `<Module>Plugin` and backend classes are
+    named `<Module>Backend`, with `<Module>` being the (camel-case) representation of the
+    Python module of the plugin without the prefix - for example, the plugin under
+    `platypush.plugins.light.hue` must be named `LightHuePlugin`.
+
+- If possible, [add a test](https://git.platypush.tech/platypush/platypush/-/tree/master/tests)
+  for the new functionality. If you have built a new functionality that works with some specific
+  device or service then it's not required to write a test that mocks the whole service, but if
+  you are changing some of the core entities (e.g. requests, events, procedures, hooks, crons
+  or the bus) then make sure to add tests and not to break the existing tests.
+
+- If the feature requires an optional dependency then make sure to document it:
+
+  - In the class docstring (see other plugins and backends for examples)
+  - In [`setup.py`](https://git.platypush.tech/platypush/platypush/-/blob/master/setup.py#L72) as
+    an `extras_require` entry
+  - In [`requirements.txt`](https://git.platypush.tech/platypush/platypush/-/blob/master/requirements.txt) -
+    if the feature is optional then leave it commented and add a one-line comment to explain which
+    plugin or backend requires it.