Added MIT License to Repo

[SethFalco]

Fixes #159

// @license       MIT

- https://github.com/victornpb/deleteDiscordMessages/blob/master/deleteDiscordMessages.user.js#L12

I noticed that there is a license specified in deleteDiscordMessages.user.js, but the license or license terms aren't made explicit anywhere in the repository.

I think it'd be nice to have a license in the root of the repository like any other open-source project.

This way it'll be:

• More consistent with other projects.
• GitHub can display the license on the project details.
• You could remove the DISCLAIMER of warranty clause from the README as that's covered by the license anyway.

(Regardless, since this is MIT licensed, I'd recommend swapping the wording out for the MIT warranty clause, or removing it from the README entirely, but that's up to you.)

More information with the issues that can come from accepting contributions without a license.

https://choosealicense.com/no-permission/

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