add notes about documentation and methods inside class

Because I always forget which way I used before

CONTRIBUTING.md

@@ -26,7 +26,95 @@ In order to resolve all difficult cases the `autopep8` is used. You can perform
 Again, the most checks can be performed by `make check` command, though some additional guidelines must be applied:
 
 * Every class, every function (including private and protected), every attribute must be documented. The project follows [Google style documentation](https://google.github.io/styleguide/pyguide.html). The only exception is local functions.
+* Correct way to document function, if section is empty, e.g. no notes or there are no args, it should be omitted:
+
+    ```python
+    def foo(argument: str, *, flag: bool = False) -> int:
+        """
+        do foo
+  
+        Note:
+            Very important note about this function
+  
+        Args:
+            argument(str): an argument
+            flag(bool, optional): a flag (Default value = False)
+  
+        Returns:
+            int: result
+  
+        Raises:
+            RuntimeException: a local function error occurs
+  
+        Examples:
+            Very informative example how to use this function, e.g.::
+  
+                >>> foo("argument", flag=False)
+  
+            Note that function documentation is in rST.
+        """
+    ```
+  
+  `Returns` should be replaced with `Yields` for generators.
+
+  Class attributes should be documented in the following way:
+
+    ```python
+    class Clazz(BaseClazz):
+        """
+        brand-new implementation of ``BaseClazz``
+  
+        Attributes:
+            CLAZZ_ATTRIBUTE(int): (class attribute) a brand-new class attribute
+            instance_attribute(str): an instance attribute
+  
+        Examples:
+            Very informative class usage example, e.g.::
+  
+                >>> from module import Clazz
+                >>> clazz = Clazz()
+        """
+  
+        CLAZZ_ATTRIBUTE = 42
+  
+        def __init__(self) -> None:
+            """
+            default constructor
+            """
+            self.instance_attribute = ""
+    ```
+
 * Type annotations are the must, even for local functions.
+* Recommended order of function definitions in class:
+
+    ```python
+    class Clazz:
+  
+        def __init__(self) -> None: ...  # replace with `__post_init__` for dataclasses
+
+        @property
+        def property(self) -> Any: ...
+
+        @classmethod
+        def class_method(cls: Type[Clazz]) -> Clazz: ...
+
+        @staticmethod
+        def static_method() -> Any: ...
+
+        def __private_method(self) -> Any: ...
+
+        def _protected_method(self) -> Any: ...
+
+        def usual_method(self) -> Any: ...
+
+        def __hash__(self) -> int: ...  # basically any magic (or look-alike) method
+    ```
+  
+  Methods inside one group should be ordered alphabetically, the only exception is `__init__` method (`__post__init__` for dataclasses) which should be defined first. For test methods it is recommended to follow the order in which functions are defined.
+
+  Though, we would like to highlight abstract methods (i.e. ones which raise `NotImplementedError`), we still keep in global order at the moment.
+
+* Abstract methods must raise `NotImplementedError` instead of using `abc.abstractmethod`. The reason behind this restriction is the fact that we have class/static abstract methods for those we need to define their attribute first making the code harder to read.
 * For any path interactions `pathlib.Path` must be used.
 * Configuration interactions must go through `ahriman.core.configuration.Configuration` class instance.
 * In case if class load requires some actions, it is recommended to create class method which can be used for class instantiating.
@@ -59,7 +147,7 @@ Again, the most checks can be performed by `make check` command, though some add
 * One file should define only one class, exception is class satellites in case if file length remains less than 400 lines.
 * It is possible to create file which contains some functions (e.g. `ahriman.core.util`), but in this case you would need to define `__all__` attribute.
 * The file size mentioned above must be applicable in general. In case of big classes consider splitting them into traits. Note, however, that `pylint` includes comments and docstrings into counter, thus you need to check file size by other tools.
-* No global variable is allowed outside of `ahriman.version` module.
+* No global variable is allowed outside of `ahriman.version` module. `ahriman.core.context` is also special case.
 * Single quotes are not allowed. The reason behind this restriction is the fact that docstrings must be written by using double quotes only, and we would like to make style consistent.
 * If your class writes anything to log, the `ahriman.core.log.LazyLogging` trait must be used.