add a document explaining mandatory coding guidelines

doc/code_style.md

@@ -5,6 +5,8 @@ The most important stuff:
 * classes use CamelCase (except where Blender needs special class names, e.g. operators)
 * methods and functions are lowercase, words separated by underscores: update_objects()
 * "private" methods/members start with a leading underscore: _internal_update()
+* Property descriptions must never end with a dot.
+* Enum identifiers should be valid Python identifiers (don't contain spaces, don't start with a number etc.)
 
 For UI texts (buttons, tooltips, panel names) we follow the Blender guidelines:
 https://wiki.blender.org/index.php/Dev:Doc/Code_Style#UI_Messages

doc/mandatory.md

@@ -0,0 +1,27 @@
+The stuff in the code_style.md document should be followed, but it is ultimately just about 
+the readability of the source code.
+The following is really important though. If these guidelines are not followed, it can cause 
+massive problems - and not just now, but also years in the future.
+
+### Enums
+
+**Always** add an index for each element. 
+If the index is missing, we can not reorder elements later and we can not rename identifiers 
+without breaking old .blend files.
+
+Wrong: 
+```python
+items = [
+    ("first_item", "First Item", "This is the first item"),
+    ("second_item", "Second Item", "This is the second item"),
+]
+```
+
+Correct:
+```python
+items = [
+    ("first_item", "First Item", "This is the first item", 0),
+    ("second_item", "Second Item", "This is the second item", 1),
+]
+```
+(note the 0 and 1 after the descriptions)