area-of-d-usage: add RISC-V for Embedded Applications section (#4248)

Author: kubo39

areas-of-d-usage.dd

@@ -157,7 +157,7 @@ $(AREA_SECTION3 $(LNAME2 embedded, Embedded applications), $(ARTICLE_FA_ICON gea
 
     Using $(HTTPS github.com/ldc-developers/ldc, LDC) (the LLVM D compiler)
     enables targeting most popular CPU architectures,
-    starting from ARM and MIPS-based embedded systems and smartphones,
+    starting from ARM, RISC-V and MIPS-based embedded systems and smartphones,
     ranging to server systems based on POWER and Sparc, and up to the "big iron" z Systems.
 
 ))