Update change log with details of release v4.21.1

Author: delphidabbler

CHANGELOG.md

@@ -6,6 +6,32 @@ Releases are listed in reverse version number order.
 
 > Note that _CodeSnip_ v4 was developed in parallel with v3 for a while. As a consequence some v3 releases have later release dates than early v4 releases.
 
+## Release v4.21.1 of 09 April 2023
+
+* Completed implementation of support for [REML version 5](https://htmlpreview.github.io/?https://github.com/delphidabbler/codesnip/blob/version-4.21.0/Docs/Design/reml.html) (ommitted from v4.20.0 in error) and fixed some bugs in the original implementation [issues #81 and #82], including:
+  * Heavily revised "active text" handling code and document model to fix support for lists introduced in v4.21.0.
+  * Added support for rendering lists in plain text reports and generated source code header comments.
+  * Added support for rendering lists in Rich Text Format for use in printed information and in reports copied to the clipboard.
+  * Overhauled HTML rendering code that generates HTML for display in the UI.
+  * Heavily revised parsing and generation of REML code.
+  * Updated "active text" validation code.
+* Prevented snippets editor from stripping REML `<p>` tags [issue #103].
+* Fixed garbled copyright symbols in generated source code [issue #80].
+* Fixed bug in code that compresses multiple white space into a single space [issue #95].
+* Fixed out of range error in code that handles text encodings [issue #97].
+* Fixed broken formatting of compiler result tables in text and rich text snippet reports & print outs [issue #101].
+* Updated copyright date displayed in about box [issue #98].
+* Updated operating system detection code to detect Windows 10/11 builds released in December 2022 and Q1 2023.
+* Some refactoring [including issue #83] 
+* Changed build process to create all files in `_build` directory and to use different zip file names [issue #78].
+* Documentation changes:
+  * Updated `Build.html` to document changes in build process.
+  * Updated `CHANGELOG.md` to fix broken link [issue #76] and to remove information about semantic versioning.
+  * Removed broken links in `Docs/License.html`.
+  * Updated copyright date in various license files [including issue #96].
+  * Fixed errors and oversights in REML documentation.
+* Removed some redundant tests that were failing due to passing invalid parameters to the revised _StrWrap_ routine [issue #79].
+
 ## Release v4.21.0 of 16 December 2022
 
 * Updated to support [REML version 5](https://htmlpreview.github.io/?https://github.com/delphidabbler/codesnip/blob/version-4.21.0/Docs/Design/reml.html) in snippet description & extra information [issue #71]: