add third param context(NULL) to mrb_parse_string()

Hello-World.md

@@ -6,8 +6,7 @@
 #include <mruby.h>
 #include <mruby/proc.h>
 #include <mruby/data.h>
-// to be replaced by #include <mruby/compile.h>
-#include <compile.h>
+#include <mruby/compile.h>
 
 int main(void)
 {
@@ -16,7 +15,7 @@ int main(void)
   char code[] = "p 'hello world!'";
   printf("Executing Ruby code from C!\n");
 
-  p = mrb_parse_string(mrb, code);
+  p = mrb_parse_string(mrb, code, NULL);
   int n;
   n = mrb_generate_code(mrb, p->tree);
   mrb_run(mrb, mrb_proc_new(mrb, mrb->irep[n]), mrb_top_self(mrb));
@@ -26,5 +25,15 @@ int main(void)
   return 0;
 }
 ```
+compile & link
+```
+$ gcc -Iinclude hello.c lib/libmruby.a -lm -o hello.out
+```
+execute
+```
+$ ./hello.out
+Executing Ruby code from C!
+"hello world!"
+```
 
 For more information on [getting started](http://matt.aimonetti.net/posts/2012/04/25/getting-started-with-mruby/), you can read this [introduction blog post](http://matt.aimonetti.net/posts/2012/04/25/getting-started-with-mruby/).