Add FPU-related build instructions

OAR/README.md

@@ -20,6 +20,11 @@ files to remove `-msoft-float` or not). The Makefile mirrors this
 need, by pointing to different paths (see FPU_SUFFIX variable),
 depending on whether you build with FPU=1 or not.
 
+Executive summary: if you want to build native-FPU-using binaries,
+you'll need to edit leon2.cfg/leon3.cfg and remove the `-msoft-float`
+before building the BSPs (that is, right after the clone from
+the RTEMS git repository - in line 87 of the script).
+
 Application sources
 -------------------
 The sources of your project must be set inside the Makefile, in the SRC