AST-1253 Add changelog

CHANGELOG.md

+# DP3 Changelog
+
+## [6.0] - 2023-08-11
+
+### New features
+
+- Optional support for Sagecal (libdirac), if that is installed.
+- Re-using model visibilities in multiple steps (e.g. ddecal steps) is now possible through the 'extradata' feature.
+- Python bindings are thoroughly refactored; entire DP3 pipelines can now be constructed from Python.
+- DP3 now has some steps with increased performance specifically for the AARTFAAC instrument.
+
+### Improvements
+
+- Under the hood some casacore structures were replaced by xtensor structures, which may reduce memory usage.
+- Deprecate the use of `FULL_RES_FLAGS`. These are not written anymore.
+- DP3 now requires EveryBeam v0.5.1
+- Experimental use of AVX and XSIMD features to speed up various operations.
+
+### Bug fixes
+
+- Fix predicting sky components with negative n-value (further than 90 degrees from phase centre) - note that this bug did not affect the demix step.