Update

CONTRIBUTING.md

-# Coding conventions
+# Adding new functions/tests
+## Interface
+- All user exposed quality checks and processing algorithms need to implement the following interface:
+  ```python
+  der yourTestFunction(
+      data: pandas.DataFrame,
+      field: str,
+      flagger: saqc.flagger.BaseFlagger,
+      *args: Any,
+      **kwargs: Any
+      ) -> (pd.DataFrame, saqc.flagger.BaseFlagger)
+  ```
+## Argument Descriptions
+
+| Name      | Description                                                                                      |
+| ----      | -----------                                                                                      |
+| `data`    | The actual dataset                                                                               |
+| `field`   | The field/column within `data`, the function is checking/processing                              |
+| `flagger` | A instance of a flagger, responsible for the translation of test results into quality attributes |
+| `args`    | Any other arguments needed to parametrize the function                                           |
+| `kwargs`  | Any other keyword arguments needed tp parametrize the function                                   |
+
+## Integrate into SaQC
+In order make your function available to the system it needs to be registered. We provide the decorator 
+[`register`](saqc/functions/register.py) in the module `saqc.functions.register`, to integrate your 
+test functions into SaQC and expose them via a name of your choice. A complete, yet useless example might
+look like that:
+
+```python
+@register("myFunc")
+def yourTestFunction(data, field, flagger, *args, **kwargs):
+    return data, flagger
+```
+
+## Example
+The function [`flagRange`](saqc/funcs/functions.py) provides a simple, yet complete implementation of the
+entire test function contribution process. You might want to look into this implementation before you start.
 
+
+# Adding a new flagger
+TODO
+
+# Testing
+SaQC comes with an extensive test suite based on [pytest](https://docs.pytest.org/en/latest/). In order to 
+run all tests use:
+```sh
+python -m pytest .
+```
+
+# Coding conventions
 ## Naming
+### Code
+We follow the follwing naming conventions
 - Classes: CamelCase
 - Functions: camelCase
 - Variables/Arguments: snake_case
+### Test Functions
+- testnames: testModule_testName
  
 ## Formatting
 We use (black)[https://black.readthedocs.io/en/stable/]
 
 ## Imports
 Only absolute imports are accepted
-
-## Testnames in configuration file
-- testnames: theModule_testName