docs: Add howto for specific architecture building

doc/building.rst

@@ -292,3 +292,5 @@ The RAM requirements of a server are mostly due to the following settings:
 - The size of the information model
 - The number of connected clients
 - The configured maximum message size that is preallocated
+
+.. include:: building_arch.rst

doc/building_arch.rst

@@ -0,0 +1,22 @@
+Building for specific architectures
+-----------------------------------
+
+The open62541 library can be build for many operating systems and embedded systems.
+This document shows a small excerpt of already tested architectures. Since the stack is only using the
+C99 standard, there are many more supported architectures.
+
+A full list of implemented architecture support can be found in the arch folder.
+
+Windows, Linux, MacOS
+^^^^^^^^^^^^^^^^^^^^^
+
+These architectures are supported by default and are automatically chosen by CMake.
+
+Have a look into the previous sections on how to do that.
+
+freeRTOS + LwIP
+^^^^^^^^^^^^^^^
+
+Credits to `@cabralfortiss <https://github.com/cabralfortiss>`_
+
+TODO