add note to the alternative fpm build instructions

Author: jalvesz

README.md

@@ -184,7 +184,8 @@ git checkout stdlib-fpm
 fpm build --profile release
 ```
 
-Or alternatively:
+**Alternative**: as fpm does not currently support fypp natively, building stdlib is done in two steps (a) launch the preprocessor through the fpm-deployment.sh script, which creates a subfolder `stdlib-fpm` (b) build the project using the processed files within the latter subfolder. Which can be done with the following commands:
+
 ```sh
 source ./ci/fpm-deployment.sh
 cd stdlib-fpm/