Add user readable summary to release notes

docs/release_notes/2.8.rst

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 Mantid Imaging 2.8
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+Mantid Imaging 2.8.0 continues to expand the capability to work with energy resolved imaging data.
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+It is now possible to do a shutter count correction when normalising transmission data. The spectrum view can do unit conversion to show and export data in wavelength, energy or time of flight, with the ability to set path length and offset. The spectrum viewer now allows zoom by holding Ctrl and dragging a box and makes it easier to change ROI colours. NaN warnings have been added to the spectrum viewer.
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+A settings dialog has been added which can be used to change themes and adjust the number of threads used for running operations. Note that themes are not well tested yet.
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+There have been many bug fixes. The live viewer is now usable with large directories. Working with ROIs is more reliable.
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+MI now has updated many dependent packages bringing performance improvements. There have been improvements to testing, type annotations and static analysis. Migration of operations to compute function style is almost complete. These changes improve stability and help developers find bugs faster.
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 New Features
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 - #2046 : Enhancement: Implement Click Dialog for Changing ROI Colors in Table