Beautify the graphs in the compilation documentation

doc/compile/README.md

@@ -249,10 +249,12 @@ link with *build/host/lib/libmruby.a*
 * create ```build/host/bin/mirb``` by compile *tools/mirb/mirb.c* and
 link with *build/host/lib/libmruby.a*
 
+```
  _____    _____    ______    ____    ____    _____    _____    ____
 | CC  |->|GEN  |->|AR    |->|CC  |->|CC  |->|AR   |->|CC   |->|CC  |
 | *.c |  |y.tab|  |core.a|  |mrbc|  |*.rb|  |lib.a|  |mruby|  |mirb|
  -----    -----    ------    ----    ----    -----    -----    ----
+```
 
 ### Cross-Compilation
 
@@ -326,6 +328,7 @@ link with *build/i386/lib/libmruby.a*
 * create ```build/i386/bin/mrbc``` by cross-compile *tools/mrbc/mrbc.c* and
 link with *build/i386/lib/libmruby_core.a* 
 
+```
  _____________________________________________________________
 |              Native Compilation for Host System             |
 | _____      ______      _____      ____      ____      _____ |
@@ -343,6 +346,7 @@ link with *build/i386/lib/libmruby_core.a*
 || *.c |    |lib.a|    |mruby|    |mirb|    |core.a|    |mrbc ||
 | -----      -----      -----      ----      ------      ----- |
  --------------------------------------------------------------
+```
 
 ## Test Environment