DOC: add human-readable CHANGELOG

Author: pavoljuhas

CHANGELOG.md

@@ -0,0 +1,41 @@
+# Release notes
+
+## Unreleased - Version 1.4.0
+
+Notable differences from version 1.3.4.
+
+### Added
+
+- Test coverage analysis at https://codecov.io/gh/diffpy/libdiffpy.
+- Make scons scripts compatible with Python 3 and Python 2.
+- Atomic numbers and possible ions for elements 99-103.
+- Define libdiffpy version requirements for client Anaconda packages.
+- Support free-standing attribute getter and setter functions.
+- `uisowidth` and `bisowidth` attributes to `ConstantPeakWidth` class.
+  This simplifies PDF modeling of structures with uniform isotropic
+  atom displacements.
+
+
+### Changed
+
+- Build Anaconda package with Anaconda C++ compiler.
+- Adopt language standard c++11.
+- Replaced boost unordered set and map types with their STL version.
+- Switch to direct serialization of `PeakWidthModel`, `PeakProfile`,
+  `PDFEnvelope`, `PDFBaseline`, and `ScatteringFactorTable` classes and
+  derived types.
+
+### Deprecated
+
+- `StructureAdapter::customPQConfig` function because it permits unexpected
+  changes in calculators setup and complicates parallel evaluation.
+
+### Removed
+
+- Virtual class inheritance of `BaseDebyeSum` and `DebyePDFCalculator`.
+
+### Fixed
+
+- Incorrect results for crystals with off-origin inversion center.
+- Incomplete `scons install` when shared library fails to compile.
+- Use of invalid iterators when removing `map` items.