[gnome-music/wip/mschraal/type-checking: 33/33] first go at docs

[Marinus Schraal <mschraal src gnome org>]

commit 06e07fb16ec36f7412cc1ccccb9924ef96f06e38
Author: Marinus Schraal <mschraal gnome org>
Date:   Tue Oct 6 11:51:01 2020 +0200

    first go at docs

 README.md | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)
---
diff --git a/README.md b/README.md
index 7af1e256..d06c9400 100644
--- a/README.md
+++ b/README.md
@@ -101,6 +101,42 @@ All non-public classwide variables or methods should be prepended with an unders
 _single_leading_underscore: weak "internal use" indicator. E.g. from M import * does not import objects 
whose name starts with an underscore.
> > >
 
+### Type checking
+
+Post 3.38 Music is starting to use type checking for all new code. This means that all arguments, returns 
values and variables have defined types and these types are checked for errors during the CI phase. Music 
uses [mypy](http://www.mypy-lang.org/) for the type checking pass.
+
+The specific syntax is best learned from the code already adapted ([coresong.py](gnomemusic/coresong.py), 
[grltrackerwrapper.py](gnomemusic/grilowrappers/grltrackerwrapper.py)) or online sources, note that Music 
uses the annotation style. A simple example follows.
+
+###### Old
+```python
+x = []
+
+x.append(1)
+```
+
+###### New
+```python
+from typing import List
+
+x: List[int] = []
+
+x.append(1)
+```
+
+#### Properties
+
+Mypy does not currently support PyGObject properties. This means property setters need to be forceibly 
ignored.
+
+```python
+@GObject.Property(type=bool, default=False)
+def selected(self) -> bool:
+    return self._selected
+
+@selected.setter  # type: ignore
+def selected(self, value: bool) -> None:
+    self._selected = value
+```
+
 ### PyGObject specifics
 
 #### Treemodel