Add 2020/.gitignore

A lot of files were made by 2020 that are not part of the repo but were
not ignored. I believe I have added them all.

2020/.gitignore

@@ -0,0 +1,20 @@
+burton/check
+burton/check_be
+burton/prog_be
+endoh2/spoiler/
+endoh3/clock
+endoh3/clock.c
+endoh3/run_clock
+ferguson2/recode
+kurdyukov2/out.gif
+kurdyukov2/out*.jpg
+kurdyukov2/output.ppm
+kurdyukov2/random.ppm
+kurdyukov2/prog_png
+kurdyukov2/prog_ppm
+kurdyukov3/prog.extra
+kurdyukov3/prog.orig
+kurdyukov4/Eugene_Onegin.txt
+kurdyukov4/Shakespeare.txt
+yang/code.c
+yang/encoded.c