docs/manual: Modify GitHub FOO_SITE explanation

GitHub has changed its layout, there is no longer any green nor grey
button, it's a link with a tiny icon that indicates if it's uploaded by
the maintainer or not.
In order to not confuse anyone, adding an image should make it clear.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-tips.txt

@@ -63,11 +63,13 @@ need to do AUTORECONF).
 
 You can see on the release page if it's an uploaded tarball or a git tag:
 
-- If there is a green download button, like
-  https://github.com/zedshaw/mongrel2/releases/tag/v1.9.1[mongrel2], then it
-  was uploaded by the maintainer and you should use the link of that button to
-  specify +FOO_SITE+, and not use the 'github' helper.
-- If there is grey download button, like
-  https://github.com/xbmc/xbmc/releases/tag/13.2-Gotham[xbmc], then it's an
-  automatically generated tarball and you should use the 'github' helper
-  function.
+image::github_hash_mongrel2.png[]
+
+- If it looks like the image above then it was uploaded by the
+  maintainer and you should use that link (in that example:
+  'mongrel2-v1.9.2.tar.bz2') to specify +FOO_SITE+, and not use the
+  'github' helper.
+
+- On the other hand, if there's is *only* the "Source code" link, then
+  it's an automatically generated tarball and you should use the
+  'github' helper function.