Add release dates to NEWS / release notes

Use ..rubric:: to add a subtitle showing the release date for releases
going back a couple of years. (Rubric works reasonably well for this,
though the vertical spacing leaves a little to be desired.)

NEWS

@@ -4,6 +4,8 @@
 Release 0.21.0
 ==============
 
+.. rubric:: 2 April 2023
+
 This release wraps htslib/samtools/bcftools version 1.17.
 
 Pysam is now compatible with Python 3.11. We have removed Python 2.x
@@ -23,9 +25,12 @@ support. Pysam is tested with Python versions 3.6 to 3.11.
 * [#1149] MacOS universal build compatibility.
 * [#1146] Fix build when CFLAGS/etc environment variables are set.
 
+
 Release 0.20.0
 ==============
 
+.. rubric:: 29 October 2022
+
 This release wraps htslib/bcftools version 1.16 and samtools version 1.16.1.
 
 * [#1113] Full compatibility with setuptools v62.1.0's build directory name changes
@@ -36,17 +41,23 @@ This release wraps htslib/bcftools version 1.16 and samtools version 1.16.1.
 Many additional type hints have been provided by the community,
 thanks!
 
+
 Release 0.19.1
 ==============
 
+.. rubric:: 27 May 2022
+
 This release wraps htslib/samtools/bcftools version 1.15.1.
 
 * [#1104] add an add_samples() method to quickly add multiple samples
   to VCF.
 
+
 Release 0.19.0
 ==============
 
+.. rubric:: 30 March 2022
+
 This release wraps htslib/samtools/bcftools version 1.15.
 
 * [#1085] Improve getopt()/getopt_long() resetting when running samtools/bcftools commands
@@ -62,20 +73,26 @@ This release wraps htslib/samtools/bcftools version 1.15.
 * Fix BGZFile.read() behaviour near or at EOF
 
 * First API for the htslib modified bases interface
-
+
+
 Release 0.18.0
 ==============
 
+.. rubric:: 17 November 2021
+
 This release wraps htslib/samtools/bcftools version 1.14.
 
 * [#1048] and [#1060], clarify documentation of index statistics with CRAM files
 * Prevent "retval may be used uninitialised" warning.
 * Add new "samples" subcommand to pysam/samtools.py
 * Introduce TupleProxyIterator iterator object class
 
+
 Release 0.17.0
 ==============
 
+.. rubric:: 30 September 2021
+
 This release wraps htslib/samtools/bcftools version 1.13. Corresponding
 to new samtools commands, `pysam.samtools` now has additional functions
 `ampliconclip`, `ampliconstats`, `fqimport`, and `version`.
@@ -118,6 +135,8 @@ Documentation improvements:
 Release 0.16.0
 ==============
 
+.. rubric:: 8 June 2020
+
 This release wraps htslib/bcftools version 1.10.2 and samtools version
 1.10. The following bugs reported against pysam are fixed due to this: