bootloader: add warning about loader segment in bootloader override example README
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The purpose of this example is to show how to override the second stage bootloader from a regular project.
-**NOTE**: Overriding the bootloader is not supported with `Makefile` build system, it is only available with `CMake`.
+**NOTE**: Functions called during the loading stage of the bootloader are expected to be placed in the iram_loader_seg to avoid being overwritten during loading. If you are overriding functions which are called during this stage then special care needs to be taken to avoid issues, e.g. by providing your own linkerscript which places the required functions in the correct sections.
## How to use example
