DOC: add human-readable CHANGELOG

Author: pavoljuhas

Diff for: CHANGELOG.md

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+# Release notes
+
+## Unreleased - Version 2017.2.1
+
+Notable differences from version 2017.1.1.
+
+### Added
+
+- Support compilation from standard git-archive tarball, i.e,
+  expand version metadata with `export-subst` git attribute.
+- Make scons scripts compatible with Python 3 and Python 2.
+- Declare compatible version requirements for client Anaconda packages.
+- Sync with upstream-objcryst v2017.2.
+
+### Changed
+
+- Remove no-op GL functions which are relevant only for fox-objcryst.
+- Anaconda package recipe to use Anaconda C++ compilers.
+- Use c++98 language standard even if Anaconda environment sets later.
+
+### Deprecated
+
+- Macro `LIBOBJCRYST_GIT_SHA` replaced with `LIBOBJCRYST_GIT_COMMIT`.
+- Member `libobjcryst_version_info::git_sha` replaced with `git_commit`.
+
+### Removed
+
+- Hard-coded diagnostic output in `SetPowderPatternObs`.
+
+### Fixed
+
+- Source typos, e.g., missed braces, misleading indentation, missing include.
+- Incomplete `scons install` when shared library fails to build.
+- `SetPowderPatternX` crash for empty vector argument.
+- Incorrectly doubled return value from `GetInversionCenter`.
+- Memory leak for a copy-constructed empty `CrystVector`.