Add note about runtime to try section of 2018/giles

Specifically point out that the invocations will take quite some time.
That is also why no script that cycles through each file.

2018/giles/README.md

@@ -20,11 +20,16 @@ make
 
 ```sh
 ./prog ioccc.txt
+
 ./prog prog.c
+
 ./prog sand.txt
+
 ./prog sand.alt.txt
 ```
 
+NOTE: these invocations will take quite some time to finish.
+
 
 ## Judges' remarks: