Cookbook: add an example of using default.do for text processing.

Documentation/cookbook/defaults/default.do

@@ -0,0 +1,25 @@
+# $1 is the target name, eg. test.txt
+# $2 in the same as $1.  We'll talk about
+#    that in a later example.
+# $3 is the temporary output file we should
+#    create.  If this script is successful,
+#    redo will atomically replace $1 with $3.
+
+if [ -e "$1.in" ]; then
+    # if a .in file exists, then do some
+    # text substitution.
+    #
+    # Remember, the user asks redo to build
+    # a particular *target* name.  It's the .do
+    # file's job to figure out what source file(s)
+    # to use to generate the target.
+    redo-ifchange "$1.in" version date
+    read VERSION <version
+    read DATE <date
+    sed -e "s/%VERSION%/$VERSION/g" \
+        -e "s/%DATE%/$DATE/g" \
+        <$1.in >$3
+else
+    echo "$0: Fatal: don't know how to build '$1'" >&2
+    exit 99
+fi