Add an example to the .cost() documentation.

README.rst

@@ -624,6 +624,10 @@ meaningful relative to one another -- more complex regexen have a
 higher cost than less complex regexen. This may be useful during
 development and optimization of regular expressions.
 
+Example::
+
+  std.log("r1 cost=" + r1.cost() + " r_alt cost=" + r_alt.cost());
+
 regex functional interface
 ==========================
 

src/vmod_re2.vcc

@@ -534,6 +534,10 @@ meaningful relative to one another -- more complex regexen have a
 higher cost than less complex regexen. This may be useful during
 development and optimization of regular expressions.
 
+Example::
+
+  std.log("r1 cost=" + r1.cost() + " r_alt cost=" + r_alt.cost());
+
 regex functional interface
 ==========================