Update changelog with new version number.

CHANGELOG.rst

 This document contains the FSL-MRS release history in reverse chronological order.
 
-WIP
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+1.1.1 (Monday 15th March 2021)
+------------------------------
 - SNR measurements should cope with negative peak amplitudes correctly
 - New metabolites added to list of default water referencing metabolites (Cr, PCr and NAA)
 - Quantification now takes into account T1 relaxation