[shotwell] Add documentation for contrast

[Jens Georg <jensgeorg src gnome org>]

commit 9d2642f7c018b0d353915f5dae4a5ccbfa9c26c0
Author: Jens Georg <mail jensge org>
Date:   Mon Dec 25 15:02:13 2017 +0100

    Add documentation for contrast

 help/C/edit-adjustments.page |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/help/C/edit-adjustments.page b/help/C/edit-adjustments.page
index d96b8fe..a388a7b 100644
--- a/help/C/edit-adjustments.page
+++ b/help/C/edit-adjustments.page
@@ -25,6 +25,10 @@
    <p>Changes the brightness to make it look like the photo was exposed for a longer or shorter time. Use 
this to correct under- or over-exposed photos.</p>
   </item>
   <item>
+   <title>Contrast</title>
+   <p>Changes the contrast of a photo. Use it to correct flat-looking photos or photos where the difference 
between bright and dark spots seems to big.</p>
+  </item>
+  <item>
    <title>Saturation</title>
    <p>Changes how vivid colors look. If your photo looks gray and washed out, try increasing the saturation. 
If colors look too bold, try decreasing it.</p>
   </item>