Document how the dirty __eq__ is called (#41)

* Document how the dirt __eq__ is called

* Move and reformat Internals docs

* fix typos

* include internals.md in mkdocs.yml

Author: Marco-Kaulea

README.md

@@ -97,3 +97,4 @@ pip install dirty-equals
 ```
 
 **dirty-equals** requires **Python 3.7+**.
+

docs/internals.md

@@ -0,0 +1,20 @@
+# Internals
+## How the magic of `DirtyEquals.__eq__` works?
+When you call `x == y`, Python first calls `x.__eq__(y)`. This would not help us
+much, because we would have to keep an eye on order of the arguments when
+comparing to `DirtyEquals` objects. But that's where were another feature of
+Python comes in.
+
+When `x.__eq__(y)` returns the `NotImplemented` object, then Python will try to
+call `y.__eq__(x)`. Objects in the standard library return that value when they
+don't know how to compare themselves to objects of `type(y)` (Without checking
+the C source I can't be certain if this assumption holds for all classes, but it
+works for all the basic ones).
+In [`pathlib.PurePath`](https://github.com/python/cpython/blob/aebbd7579a421208f48dd6884b67dbd3278b71ad/Lib/pathlib.py#L751)
+you can see an example how that is implemented in Python.
+
+> By default, object implements `__eq__()` by using `is`,
+> returning `NotImplemented` in the case of a false comparison: 
+> `True if x is y else NotImplemented`.
+
+See the Python documentation for more information ([`object.__eq__`](https://docs.python.org/3/reference/datamodel.html#object.__eq__)).

mkdocs.yml

@@ -44,6 +44,7 @@ nav:
       - types/boolean.md
       - types/other.md
       - types/custom.md
+  - Internals: internals.md
 
 markdown_extensions:
   - toc: