Update documentation on XDG Base Directory specification support

Signed-off-by: Stanisław Wysocki <garethel@protonmail.com>

doc/rules.txt

@@ -1,12 +1,17 @@
 Rules System
 ============
-The Timewarrior rule system reads your ~/.timewarrior/timewarrior.cfg file and
+The Timewarrior rule system reads your timewarrior.cfg file and
 uses the combination of configuration settings and logic within to:
 
 - Define configuration and customization details
 - Define tags, exclusions, constraints
 - Define various policies
 
+On non-Unix systems config file is expected in ~/.timewarrior directory.
+On Unix systems, if legacy ~/.timewarrior directory doesn't exists, config is
+read from $XDG_CONFIG_HOME/timewarrior directory (if not specified,
+$XDG_CONFIG_HOME defaults to ~/.config).
+
 The rules are a mechanism to apply late-bound logic and data to various
 functions. Whenever data changes, the rule system is run, which will run each
 rule in turn, if it applies, going from top to bottom in the rules file. There
@@ -19,8 +24,8 @@ will initially be minimal, but grow to be come more capable.
 
 Format
 ------
-The rules are written as UTF8 text in the ~/.timewarrior/timewarrior.cfg text
-file. Other rules files may be included:
+The rules are written as UTF8 text in the timewarrior.cfg text file. Other
+rules files may be included:
 
   import /path/to/other/rule/file