Add 'watch' task.

Gemfile

@@ -4,3 +4,5 @@ gem "rake", "~> 13.0"
 gem "asciidoctor-pdf", "~> 1.5"
 
 gem "asciidoctor-diagram", "~> 2.1"
+
+gem "listen", "~> 3.4"

Gemfile.lock

@@ -27,7 +27,11 @@ GEM
     concurrent-ruby (1.1.8)
     css_parser (1.7.1)
       addressable
+    ffi (1.15.0)
     hashery (2.1.2)
+    listen (3.4.1)
+      rb-fsevent (~> 0.10, >= 0.10.3)
+      rb-inotify (~> 0.9, >= 0.9.10)
     pdf-core (0.7.0)
     pdf-reader (2.4.1)
       Ascii85 (~> 1.0.0)
@@ -51,6 +55,9 @@ GEM
       prawn (~> 2.2)
     public_suffix (4.0.6)
     rake (13.0.3)
+    rb-fsevent (0.10.4)
+    rb-inotify (0.10.1)
+      ffi (~> 1.0)
     rexml (3.2.4)
     ruby-rc4 (0.1.5)
     safe_yaml (1.0.5)
@@ -65,6 +72,7 @@ PLATFORMS
 DEPENDENCIES
   asciidoctor-diagram (~> 2.1)
   asciidoctor-pdf (~> 1.5)
+  listen (~> 3.4)
   rake (~> 13.0)
 
 BUNDLED WITH

Rakefile

@@ -270,6 +270,20 @@ task :htmldocs => [ directory(htmldir), *htmldocs ]
 
 task :pdfdocs => [ directory(pdfdir), *pdfdocs ] 
 
+desc "Watch this directory for changes and rebuild on changes."
+task :watch do
+  require 'listen'
+  listener = Listen.to('.') do |modified, added, removed|
+    if modified.length > 0 || added.length > 0
+      Rake::Task[:default].invoke
+      Rake.application.tasks.each { |t| t.reenable }
+    end
+  end
+
+  listener.start
+  sleep
+end
+
 task :init => [
   directory(htmldir),
   directory(pdfdir),