Add migration guide and changelog
Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/docs/3.0-migration-guide.d/split_config.md b/docs/3.0-migration-guide.d/split_config.md
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+Introduce a level of indirection and versioning in the config files
+-------------------------------------------------------------------
+
+`config.h` was split into `build_info.h` and `mbedtls_config.h`.
+`build_info.h` is intended to be included from C code directly, while
+`mbedtls_config.h` is intended to be edited by end users whishing to
+change the build configuration, and should generally only be included from
+`build_info.h`. This is because all the preprocessor logic has been moved
+into `build_info.h`, including the handling of the `MBEDTLS_CONFIG_FILE`
+macro.
+
+Mandatory version symbols were introduced for `MBEDTLS_CONFIG_FILE` and
+`MBEDTLS_USER_CONFIG_FILE`, `MBEDTLS_CONFIG_VERSION` and
+`MBEDTLS_USER_CONFIG_VERSION` respectively. Both config files should include
+a definiton of their respective version symbol, with a value of `1` to be
+considered valid.