Add a coding convention file

doc/coding_conventions.md

+# Coding conventions
+
+How to style your C and Ruby code which you want submit to mruby.
+
+## C code
+
+The core part (parser, bytecode-interpreter, core-lib, etc.) of mruby is written in the C programming language. Please note the following hints for your C code:
+
++ comply with C99 [ISO/IEC 9899:1999]
++ don't use C++ style comments, only use /* ... */
++ reduce library dependencies to a minimum
++ insert a break after the method return value:
+
+    int
+    main(void)
+    {
+      ...
+    }
+
+## Ruby code
+
+Parts of the standard library of mruby is written in the Ruby programming language itself. Please note the following hints for your Ruby code:
+
++