Added release notes for MSlice

[SilkeSchomann]

Description of work

This PR adds the release notes for MSlice for release 6.12. 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 these are the release notes :-)

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[GuiMacielPereira]

The notes were already grammatically correct, I just made a few suggestions on how I prefer release notes to be written. This reminds me that I still haven't organised a discussion on the phrasing of release notes, which I should do at the beginning of next release, so we can all agree on the same kind of phrasing.

[GuiMacielPereira]

Thanks for accepting my suggestions 👍