DOC: add human-readable CHANGELOG

Author: pavoljuhas

CHANGELOG.md

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+# Release notes
+
+## Unreleased - Version 3.0.0
+
+Notable differences from version 1.3.5.
+
+### Added
+
+- Compatibility with Python 3.7, 3.6, 3.5 in addition to 2.7.
+- Aliases for 17 non-standard space group names from cctbx.
+- Support for intersphinx links to Python and NumPy documentation.
+- Dependency and use of the `six` PY2/PY3 compatibility package.
+- Documentation hosting at readthedocs.org.
+
+### Changed
+
+- Rename the package and all its module names to lowercase.
+- Use UTF-8 encoding when writing structure files.
+- Refactor parsing of XCFG format.  Avoid use of generated code.
+- Refactor all starred imports to explicit so they can be checked.
+- Adopt napoleon style for docstrings.
+- Update docstrings for `Atom`, `Lattice`, `SymOp`, `SpaceGroup`.
+- Switch to platform-independent "noarch" Anaconda package.
+
+### Deprecated
+
+- Old camel case module names such as `diffpy.Structure`.
+- Variable `__gitsha__` in the `version` module which was renamed
+  to `__git_commit__`.
+
+### Removed
+
+- Unused exception `IsotropyError`.
+- Unused class `BRAtomsStructure` and associated parser.
+
+### Fixed
+
+- Loading of empty CIF files with no specified sites.
+- Parsing of CIFs with `?` value for unknown displacement parameters.
+- Symmetry constraint equations for ADPs so they avoid self-reference.
+- Use `StructureFormatError` exception for CIF with unknown space group.
+- Open files within the `with` context so they get closed when done.
+- Invalid escape sequences in string values.