docs: Add section Technical restrictions (#1052)

Author: lord-haffi

docs/index.rst

@@ -31,14 +31,15 @@ Contents
    :maxdepth: 2
 
    Fundamentals <fundamentals>
+   Technical restrictions <restrictions>
    Data Model Overview <api/modules>
    BusinessObjects <api/bo4e.bo>
    Components <api/bo4e.com>
    Enumerations <api/bo4e.enum>
    Versioning <versioning>
    Release Workflow <release_workflow>
-   Contributing <contributing_guide>
    Changelog <changelog>
+   Contributing <contributing_guide>
    Authors <authors>
    License <license>
 

docs/restrictions.rst

@@ -0,0 +1,26 @@
+======================
+Technical restrictions
+======================
+
+There are some technical restrictions throughout the BO4E data models which are important if you want to use it.
+There are several reasons for these but mostly due to compatibility reasons as we want to design the data model
+such that it can be implemented in most programming languages.
+
+* The data model is generally not object oriented. The reference implementation (BO4E-Python) uses inheritance
+  to save some code but the generated JSON schemas do not contain any information about inheritance as it is not
+  supported by the official JSON schema structure. As a result, the following constraints apply:
+
+  * You can only differentiate BOs and COMs by using the enums `BoTyp` and `ComTyp`. They are not directly
+    referenced from the data model but are rolled out to support developers if they need to differentiate the
+    types.
+  * There will be some redundancy in terms of field definitions.
+
+* In general, BOs won't be referenced by COMs but it's not prohibited. In fact, there are some cases in which a COM
+  references a BO. Keep that in mind when designing a package structure e.g. to prevent circular import errors.
+* There are no circular references. This means, when drawing a reference graph of BO4E there will be no loops.
+
+  * As a result, we cannot directly support `n x m`-relationships as they would require a back-reference.
+    If you're designing a database structure you have to manually add these relationship tables where needed.
+
+* We won't use a `Union`-type which is represented in JSON schemas as `anyOf` except for a nullable type. I.e.
+  the type can never be a union of two or more types which are not `null`.