Prepare release 0.21.1 (#457)

docs/CHANGELOG.md

@@ -1,9 +1,27 @@
 Changelog
 =========
 
+Version 0.21.1
+--------------
+
+January 4th, 2023
+
+- `Particle` class and submodules:
+  - Better test coverage.
+- `PDGID` class method and related standalone functions:
+  - Minor code simplifications.
+  - Significant improvements to test coverage.
+- Documentation:
+  - README: info added on how to create user-defined particles.
+- Tests:
+  - Various improvements to the CI.
+  - Updates to pre-commit hooks and CI YAML files.
+
 Version 0.21.0
 --------------
 
+November 8th, 2022
+
 - Data files:
   - Added the 2022 PDG data table file, now default.
   - Version 11 of package CSV data files:
@@ -28,6 +46,8 @@ Version 0.21.0
 Version 0.20.1
 --------------
 
+March 29th, 2022
+
 - `Particle` class:
   - More tests of PDGID functions for special particles.
 - Experiment-specific modules:
@@ -42,6 +62,8 @@ Version 0.20.1
 Version 0.20.0
 --------------
 
+January 15th, 2022
+
 **First Python 3 (3.6+) only version.**
 
 - `Particle` class: