Add doc/WritingTests.txt to distribution

doc/include.am

@@ -37,7 +37,9 @@ endif
 EXTRA_DIST+= doc/HACKING doc/asciidoc-helper.sh			\
              $(html_in) $(man_in) $(txt_in)			\
              doc/state-contents.txt				\
-             doc/torrc_format.txt doc/TUNING
+             doc/torrc_format.txt 				\
+	     doc/TUNING						\
+	     doc/WritingTests.txt
 
 docdir = @docdir@