yotta Readme: expand configuration example

yotta/data/README.md

@@ -30,11 +30,20 @@ mbed TLS makes it easy to disable any feature during compilation that isn't requ
 
 The list of available compilation flags is presented in the fully documented [config.h file](https://github.com/ARMmbed/mbedtls/blob/development/include/mbedtls/config.h), present in the `mbedtls` directory of the yotta module.
 
-If you need to adjust those flags, you can provide your own configuration file with suitable `#define` and `#undef` statements. These will be included between the default definitions and the sanity checks. Your configuration file should be in your application's `include` directory, and can be named freely; you just need to let mbed TLS know the file's name. To do that, use yotta's [configuration system](http://docs.yottabuild.org/reference/config.html). The file's name should be in your `config.json` file, under mbedtls, as the key `user-config-file`. For example:
+If you need to adjust those flags, you can provide your own configuration-adjustment file with suitable `#define` and `#undef` statements. These will be included between the default definitions and the sanity checks. Your configuration file should be in your application's include directory, and can be named freely; you just need to let mbed TLS know the file's name. To do that, use yotta's [configuration system](http://docs.yottabuild.org/reference/config.html). The file's name should be in your `config.json` file, under mbedtls, as the key `user-config-file`.
+
+For example, in an application called `myapp`, if you want to enable the EC J-PAKE key exchange and disable the CBC cipher mode, you can create a file named for example `mbedtls-config-changes.h` in the `myapp` directory containing the following lines:
+
+    #define MBEDTLS_ECJPAKE_C
+    #define MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED
+
+    #undef MBEDTLS_CIPHER_MODE_CBC
+
+and then create a file named `config.json` at the root of your application with the following contents:
 
     {
        "mbedtls": {
-          "user-config-file": "\"myapp/my_mbedtls_config_changes.h\""
+          "user-config-file": "\"myapp/mbedtls-config-changes.h\""
        }
     }