Develop a Release Notes Template

[bushrat011899]

Proposal

When stubbing out new release notes, an approachable template should be provided instead of a simple TODO. This will help set the tone for these individual pieces for new authors, and provide a skeleton to help overcome writers block.

As an example of such a template:

<!-- TODO -->
<!-- Please use the below template as a guide, but do not feel constrained to its exact layout -->

Open with sentence providing clear context for the changes about to be described.

Follow with an example of what Bevy was like prior to this release, taking care to highlight any key differences that can illustrate why we are making this change.

Consider including example code using code blocks

```rust
use bevy::prelude::*;

fn main() {
    App::new().run();
}
```

Where appropriate, [link to external information](https://bevy.org/) on related topics.

And if this change is appropriately visual, please include imagery!

![Alt text for this image](a_cool_image.png)

End with a short call to action linking to further information, and any possible related work that may come in the future.

I imagine the simplest way to do this automatically would be to amend the generate_release_notes function to inject whatever starting template is appropriate. Most of the work of this issue would likely be on the creation of the template itself.

An example of previous instructions given during the creation of release notes can be found in the 0.12 announcement.

Alternatives

Perhaps including a single Example.md, or amending the contributors guide could also serve this purpose?

This was originally discussed in the 0.14 release crew working group