smal coding_convention update

doc/coding_conventions.md

@@ -10,7 +10,7 @@ The core part (parser, bytecode-interpreter, core-lib, etc.) of mruby is written
 
 mruby should be highly portable to other systems and compilers. For that it is recommended to keep your code as close as possible to the C99 standard (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf).
 
-Although we target C99, VC is also a important target for mruby, so that we avoid local variable declaration in the middle.
+Although we target C99, VC is also an important target for mruby, so that we avoid local variable declaration in the middle.
 
 ### Reduce library dependencies to a minimum
 
@@ -20,6 +20,8 @@ The dependencies to libraries should be put to an absolute minimum. This increas
 
     /* This is the prefered comment style */
 
+Use C++ style comments only for temporary comment e.g. commenting out some code lines.
+
 ### Insert a break after the method return value:
 
     int