docs: Changelog added.

CHANGELOG.md

+# Changelog
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+
+## [Unreleased]
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+
+### Bug Fixes
+- PBMod updated according to Table 5 of ESS-0038258 revision 6.
+- PBDest refers to Table 8 of ESS-0038258 revision 6.
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+
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+### Documentation Changes
+- Changelog added.
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+
+## Version 1.5.1+1 (2020-03-25)
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+## Version 1.5.1 (2020-03-24)
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+
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+## Version 1.5.0 (2020-03-17)
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