.gitignore: remove editor- and OS-specific files

Whenever Vim opens a file, it creates a .<filename>.swp file in the same
directory as the file. Emacs adds *~ backup files. macOS creates
.DS_Store files. Other editors and OS' surely do their own thing. Rather
than add each one's own swap/backup file to this .gitignore, encourage
users to add the corresponding items to their system-wide gitignore
files.

Change-Id: I5535f5d2f1ebe896eef108cfda087dcb4c50f031
Signed-off-by: Matthew DeVore <matvore@gmail.com>

.gitignore

@@ -1,6 +1,9 @@
 /.project
 /target
-.DS_Store
 infer-out
 bazel-*
-*~
+
+# Do not add editor- and OS-specific files like *~ (Emacs) and .DS_Store
+# (macOS). Instead, add them to $XDG_CONFIG_HOME/git/ignore
+# (~/.config/git/ignore if $XDG_CONFIG_HOME is not set) or
+# $GIT_DIR/info/exclude. See "git help gitignore" for details.