[pitivi] Ignore files generated when running "make html"

[Thibault Saunier <tsaunier src gnome org>]

commit e98cccc969035c24bd195500fa99d8c825f79571
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Wed Apr 6 13:49:19 2016 +0200

    Ignore files generated when running "make html"
    
    Reviewed-by: Thibault Saunier <tsaunier gnome org>
    Differential Revision: https://phabricator.freedesktop.org/D892

 .gitignore |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/.gitignore b/.gitignore
index 734e22d..6f456a6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -55,6 +55,10 @@ po/.intltool-merge-cache
 po/pitivi.pot
 tests/samples/*.proxy.mkv
 
+# Generated by make html
+help/html/
+help/index.cache
+
 # Generated when running the project
 *.pyc