Added release notes for MSlice for 6.13

[SilkeSchomann]

Description of work

This PR adds the release notes for MSlice for release 6.13. of Mantid.

There is no associated issue.

To test:

Read the release notes and check grammar as well as spelling.

This does not require release notes because it is the release notes :-;

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Reviewer

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Code Review

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• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
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• Do the release notes conform to the release notes guide?

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[GuiMacielPereira]

These are my suggestions regarding release notes, although they're more of a stylistic choice.