Update README with OS Integration Details, exclude them from main Bazel project

Since the OS integrations are currently realized as separate Bazel projects, and contain individual documentation, this should be reflected in the main README. Also, the sub-directories should be excluded explicitly from the top-level Bazel project. For this, the https://bazel.build/run/bazelrc#bazelignore is updated.

Author: opajonk

.bazelignore

@@ -1 +1,3 @@
+autosd
+ebclfsa
 qnx_qemu

README.md

@@ -57,6 +57,16 @@ Execute `bazel query //feature_showcase/...` to obtain list of targets that You
 bazel build --config bl-x86_64-linux @score_orchestrator//src/... --verbose_failures
 ```
 
+## Operating system integrations
+
+> [!NOTE]
+> Integrations of Eclipse S-CORE into reference operating systems are currently realized as **independent Bazel projects**.
+> Please refer to the README documents in the respective sub-directories for details about the specific integration.
+
+* [QNX](./qnx_qemu/README.md)
+* [Red Hat AutoSD](./autosd/build/README.md)
+* [Elektrobit corbos Linux for Safety Applications](./ebclfsa/README.md)
+
 ## Known Issues ⚠️
 
 ### Orchestrator