Add note concering content of extras/precompile (only relevant for maintainers).

Author: aentinger

extras/precompile/README.md

@@ -1,6 +1,9 @@
 :floppy_disk: Precompile `open62541` library for `cortex-m7`
 ============================================================
 The following instructions can be used to pre-compile the `open62541` library for `cortex-m7`.
+
+**Note**: these steps are only relevant for the maintainers of this library.
+
 ```bash
 ./docker-build.sh
 ./docker-run.sh