docs/manual: cleanup github helper docs

Explicitly state that the github helper should not be used when there is
a release tarball.

Properly render the list by separating it from the previous paragraph.

[Peter: fix typo as pointed out by Maxime]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-tips.txt

@@ -61,10 +61,11 @@ may be slightly different (e.g. it contains a configure script so we don't
 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+.
+  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