prepare release `0.18.0` and switch to the recommended license format…

[AngelOnFira]

These should be the changes since v0.17.0

## What's Changed
* xmac94x/GitHub actions by @xMAC94x in https://github.com/amethyst/specs/pull/749
* The scope of the unsafe block can be appropriately reduced by @peamaeq in https://github.com/amethyst/specs/pull/747
* update all depdnencies including hashbrown by @xMAC94x in https://github.com/amethyst/specs/pull/748
* Added an extra example to the sixth chapter in the book by @codedcosmos in https://github.com/amethyst/specs/pull/745
* Enable fmt by @xMAC94x in https://github.com/amethyst/specs/pull/752

## New Contributors
* @peamaeq made their first contribution in https://github.com/amethyst/specs/pull/747
* @codedcosmos made their first contribution in https://github.com/amethyst/specs/pull/745

**Full Changelog**: https://github.com/amethyst/specs/compare/v0.17.0...v0.18.0

Can you mention the other changes that aren't in the readme yet?

[xMAC94x]

Not a huge fan of just printing the full merge history:

• unsafe block scope is nice, but has no external influence, its just cleanup
• extra examples are also not externally important, when someone is deciding wether they wanna upgrade or not it doesnt matter.
  • even the github actions technically dont fall in that category, i am tempting to remove them.
  • fmt is also just internal stuff, no consumer of the api cares about that.

New contributors would be funny, but at this point of the crate it was never done in history and i dont know if we should start with it and bloat our Changelog with it.

The link to the full changelog sounds doable, i could include that