Update release notes.

Author: Ayla Khan

src/ReleaseNotes.txt

@@ -1,6 +1,22 @@
 Release Notes
 =============
 
+Version 2.3.1
+-------------
+Bug fix release
+(https://github.com/SCIInstitute/Seg3D/issues?q=milestone:2.3.1).
+
+* #75: Pad tool
+* #107: NIfTI file exporter
+* #108: ITK layer export does not cast image types
+* #126: exposing the group of a layer
+* #138: Layer name overrides user-provided file name for non-DICOM file series export
+* #141: Threshold on a single value
+
+Version 2.3.0
+-------------
+This release introduces the re-designed speedline tool.
+
 Version 2.2.1
 -------------
 This version is a primarily a bug fix release for the IBBM course.