Add to FAQ about seeing fix changes

It seems to me that it is a good idea to show people how to see what
changed in order to get an entry to work in modern systems. I gave an
example where it's harder to tell because it's a very long line and an
example where it's easier to tell as it's not long lines but rather
shorter lines. Well it's easier to see on a line-by-line basis ..
whether it's easy to see what actually changed is probably another
matter entirely (especially since it's easier to see when you have
compiler errors in front of you) :-)

faq.md

@@ -125,6 +125,45 @@ does not. We're not sure how this is compatibility but either way it can cause a
 problem and it is this that has complicated most of the fixes though again some
 can look almost identical.
 
+## Q: How can I easily see what was changed in order to get an entry to work in modern systems?
+
+Although the [thanks-for-fixes.md](/thanks-for-fixes.md) file sometimes gives
+commands to tell you how to do this, in general you can do:
+
+```sh
+diff entry.c entry.orig.c
+diff prog.c prog.orig.c
+```
+
+to see it as if you wanted to make it go back to the original and, to see the
+difference to fix it, you can do:
+
+```sh
+diff entry.orig.c entry.c
+diff prog.orig.c prog.c
+```
+
+For instance to see how [2001/anonymous](2001/anonymous/README.md) was fixed you
+can do:
+
+```sh
+diff 2001/anonymous/anonymous.orig.c 2001/anonymous/anonymous.c
+```
+
+You might be quite surprised how little some entries had to be changed and at
+the same time how much other entries had to be changed, often with quite complex
+differences! In some cases if the line is rather long, like the above mentioned
+one, it will be harder to see what changed but in other cases like
+[1984/decot](1984/decot/README.md) it's a lot easier:
+
+```sh
+diff 1984/decot/decot.orig.c 1984/decot/decot.c
+```
+
+Well, at least it's easier see the differences on a line-by-line basis but maybe
+not what actually changed, especially since it's easier to know what was fixed
+when you have compiler errors :-)
+
 
 ## Q: I cannot get entry XYZZY from year 19xx to compile!