[geary] Add new help page to the build.

[Michael Gratton <mjog src gnome org>]

commit 4a0c458f0caa497098e1fd055744b106bd9232a1
Author: Michael James Gratton <mike vee net>
Date:   Wed Jun 13 22:40:22 2018 +1000

    Add new help page to the build.

 help/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/help/meson.build b/help/meson.build
index 5b4b3829..ff36035e 100644
--- a/help/meson.build
+++ b/help/meson.build
@@ -2,6 +2,7 @@ geary_help_pages = [
   'accounts.page',
   'archive.page',
   'bugs.page',
+  'contributing.page',
   'index.page',
   'label.page',
   'limits.page',