Fixes in 2018/bellard

Fix command in README.md for macOS:

    -preview lena.ppm # On MacOS
    +open -a Preview lena.ppm # On macOS

Move warning that the program does not protect against invalid input to
the top of the author's comments (seems the best place).

Format fixes.

Add to .gitignore files that are generated by suggested commands.

2018/bellard/.gitignore

@@ -1,3 +1,6 @@
 lena.ppm
 prog
 small.jpg
+fruits.ppm
+lena512.ppm
+vintage_cars.ppm