The most obvious way to do this is to set the post date far into the future, so it always displays above any other posts, including ones which you may make in the future.
This is, fortunately, not hard to do. You can change the post date on any blog post, any time you feel the need.
Thanks to the ability to schedule the publishing of posts in the future, publishing a Welcome post with a future date is just slightly complicated. You'll have to add the Index / Welcome post, in two steps.
- Publish the Index / Welcome post, as normally.
- Edit the Index / Welcome post, and give it a date well into the future.