Comment .gitignore

Add some header comments and an introductory comment explaining how to
check that no rule ignores files present in the repository.

Signed-off-by: Love Hornquist Astrand <lha@h5l.org>

.gitignore

@@ -1,9 +1,12 @@
-# git-ls-files --others --exclude-from=.git/info/exclude
-# Lines that start with '#' are comments.
-# For a project mostly in C, the following would be a good set of
-# exclude patterns (uncomment them if you want to use them):
-# *.[oa]
-# *~
+# After changing this file, please run:
+#
+#     git ls-files -i --exclude-standard
+#
+# to check that you haven't inadvertently ignored any tracked files.  This
+# command should return no output.  Any files listed by it are files
+# present in the repository but ignored by .gitignore.
+
+# General rules, with some exclusions for where they're too general.
 
 .DS_Store
 .deps/
@@ -31,6 +34,8 @@ asn1_*.[cx]
 *.lo
 *.o
 
+# Top-level files.
+
 /aclocal.m4
 /autom4te.cache
 /compile
@@ -46,6 +51,8 @@ asn1_*.[cx]
 /missing
 /ylwrap
 
+# Files in subdirectories.
+
 /admin/ktutil
 /appl/afsutil/afslog
 /appl/afsutil/pagsh