DOC: Update changelog and explain `match_newer_than_file`

CHANGELOG.md

@@ -23,6 +23,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   variables to `int`, `float`, `str` and `bool`.
 - Transformer function definition in the cfood support variable
   substitutions now.
+- `match_newer_than_file` option for `DirectoryConverter`: A reference
+  file containing (only) an ISO-formatted datetime string can be
+  specified here. Directories with this option won't match if all
+  their contents were last modified before that datetime.
 
 ### Changed ###
 

src/doc/converters/standard_converters.rst

@@ -6,9 +6,17 @@ These are the standard converters that exist in a default installation.  For wri
 
 Directory Converter
 ===================
-The Directory Converter creates StructureElements for each File and Directory
-inside the current Directory. You can match a regular expression against the
-directory name using the 'match' key.
+
+The Directory Converter creates StructureElements for each File and
+Directory inside the current Directory. You can match a regular
+expression against the directory name using the 'match' key.
+
+With the optional ``match_newer_than_file`` key, a path to file
+containing only an ISO-formatted datetime string can be specified. If
+this is done, a directory will only match if it contains at least one
+file or directory that has been modified since that datetime. If the
+file doesn't exist or contains an invalid string, the directory will
+be matched regardless of the modification times.
 
 Simple File Converter
 =====================