Expand GDScript guidelines with guidelines from the main repository.

Author: Ivorforce

engine/guidelines/gdscript_language_guidelines.rst

@@ -1,6 +1,21 @@
 GDScript language design guidelines
 ===================================
 
+GDScript is a tightly designed language: Features are added because they are needed,
+and not because they can be added or are interesting to develop. For more information,
+please refer to :ref:`doc_best_practices_for_engine_contributors`.
+
+Typing guidelines
+-----------------
+
+GDScript is gradually typed. Type hints are optional and help with static analysis and performance.
+However, typed code must easily interoperate with untyped code.
+gIn addition, we expect new typing features to be guaranteed by Godot core. While other
+languages can rely on static typing alone to ensure type correctness, Godot exposes
+lots of ways to interact with the data such that this assumption might fail.
+Therefore, new typing features need to be guaranteed by core in order to be useful to
+GDScript.
+
 Annotation guidelines
 ---------------------