Dissuade .gitignore references to personal tools

Developers run all sorts of different tools within Git repositories,
any of which can leave their own special trashfiles all over the
place. We can't every hope to catalog them all, so better to
recommend developers simply configure a global core.excludesfile to
filter the irrelevant files which tend to get created by their
personal choice of tools. Add a comment block explaining this, for
clarity, and remove the one current editor-specific entry present.

We can, and should of course, continue to list files created by the
tools recommended by our workflow (test frameworks, documentation
and packaging builds, et cetera).

This change is a port of Ib58a57267b064e4142686de6c37a70dbff04b9a7
from the openstack-dev/cookiecutter repository.

Change-Id: I2a8e2379075a7f9860d7ef5b64dad87196f155f5

.gitignore

@@ -1,3 +1,14 @@
+# Add patterns in here to exclude files created by tools integrated with this
+# repository, such as test frameworks from the project's recommended workflow,
+# rendered documentation and package builds.
+#
+# Don't add patterns to exclude files created by preferred personal tools
+# (editors, IDEs, your operating system itself even). These should instead be
+# maintained outside the repository, for example in a ~/.gitignore file added
+# with:
+#
+# git config --global core.excludesfile '~/.gitignore'
+
 pkg/
 Gemfile.lock
 vendor/
@@ -6,7 +17,6 @@ spec/fixtures/
 .bundle/
 coverage/
 .idea/
-*.swp
 *.iml
 /openstack
 /log