configs/README.txt: Improve example with MBEDTLS/TF_PSA_CRYPTO_CONFIG_FILE

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/configs/README.txt b/configs/README.txt
index 9e47134..38348dd 100644
--- a/configs/README.txt
+++ b/configs/README.txt
@@ -15,12 +15,14 @@
 1. Replace the default files include/mbedtls/mbedtls_config.h and
    tf-psa-crypto/include/psa/crypto_config.h with the chosen ones.
 
-2. Use the MBEDTLS_CONFIG_FILE and TF_PSA_CRYPTO_CONFIG_FILE options of the
-   CMake build system:
+2. Use the MBEDTLS_CONFIG_FILE and TF_PSA_CRYPTO_CONFIG_FILE CMake options. For
+   example, to build out-of-tree with the config-ccm-psk-tls1_2.h and
+   crypto-config-ccm-psk-tls1_2.h configuration pair:
 
-   cmake -DMBEDTLS_CONFIG_FILE="path-to-your-mbedtls-config-file" \
-         -DTF_PSA_CRYPTO_CONFIG_FILE="path-to-your-tf-psa-crypto-config-file" .
-   make
+   cmake -DMBEDTLS_CONFIG_FILE="configs/config-ccm-psk-tls1_2.h" \
+         -DTF_PSA_CRYPTO_CONFIG_FILE="configs/crypto-config-ccm-psk-tls1_2.h"
+         -B build-psktls12 .
+   cmake --build build-psktls12
 
 The second method also works if you want to keep your custom configuration
 files outside the Mbed TLS tree.