Explain where the .meta packages are located and how to edit them (#868)

* Explain where the .meta packages are located and how to edit them

* Apply suggestions from code review

* Update README.md

Co-authored-by: Pablo Garrido <[email protected]>

Author: amilcarlucas

README.md

@@ -173,6 +173,8 @@ An example of a micro-ROS application using PlatformIO is available [here](https
 
 ## How to build the precompiled library
 
+If you need to add custom packages or types, or customize any internal parameter of the micro-ROS stack, you will need to recompile this library from source code:
+
 ```bash
 docker pull microros/micro_ros_static_library_builder:galactic
 docker run -it --rm -v $(pwd):/project --env MICROROS_LIBRARY_FOLDER=extras microros/micro_ros_static_library_builder:galactic
@@ -186,7 +188,10 @@ docker run -it --rm -v $(pwd):/project --env MICROROS_LIBRARY_FOLDER=extras micr
 
 Available targets `LIBRARY_TARGETS` are available on the [top of the extras/library_generation/library_generation.sh file](https://github.com/micro-ROS/micro_ros_arduino/blob/main/extras/library_generation/library_generation.sh#L13-L24)
 
-Note that folders added to `extras/library_generation/extra_packages` and entries added to `extras/library_generation/extra_packages/extra_packages.repos` will be taken into account by this build system.
+Folders added to `extras/library_generation/extra_packages` and entries added to `extras/library_generation/extra_packages/extra_packages.repos` will be taken into account by this build system.
+This should be used for example when adding custom messages types or custom micro-ROS packages.
+
+You can [configure many parameters](https://micro.ros.org/docs/tutorials/advanced/microxrcedds_rmw_configuration/) of the library by editing the respective `.meta` file in the `extras/library_generation/` directory.
 
 ## Patch Arduino board for support precompiled libraries
 ### Patch Teensyduino