DOC: Update changelog

Author: effigies

Changelog

@@ -25,6 +25,41 @@ Eric Larson (EL), Demian Wassermann, and Stephan Gerhard.
 
 References like "pr/298" refer to github pull request numbers.
 
+3.0.0 (To Be Determined)
+========================
+
+New features
+------------
+* GiftiImage method ``agg_data()`` to return usable data arrays (pr/793)
+  (Hao-Ting Wang, reviewed by CM)
+* Accept ``os.PathLike`` objects in place of filenames (pr/610) (Cameron
+  Riddell, reviewed by MB, CM)
+* Function to calculate obliquity of affines (pr/815) (Oscar Esteban,
+  reviewed by MB)
+
+Enhancements
+------------
+* Improve clarity of coordinate system tutorial (pr/823) (Egor Panfilov,
+  reviewed by MB)
+
+Bug fixes
+---------
+* Re-import externals/netcdf.py from scipy to resolve numpy deprecation
+  (pr/821) (CM)
+
+Maintenance
+-----------
+* Support Python >=3.5.1, including Python 3.8.0 (pr/787) (CM)
+* Manage versioning with slightly customized Versioneer (pr/786) (CM)
+* Reference Nipy Community Code and Nibabel Developer Guidelines in
+  GitHub community documents (pr/778) (CM, reviewed by MB)
+
+API changes and deprecations
+----------------------------
+* Deprecate ``DataobjImage.get_data()`` API, to be removed in nibabel 5.0
+  (pr/794, pr/809) (CM, reviewed by MB)
+
+
 2.5.1 (Monday 23 September 2019)
 ================================