[web] Add link to signing on downloads page

[LEDE Commits]

jow pushed a commit to web.git, branch master:
https://git.lede-project.org/?p=web.git;a=commitdiff;h=dc1ec1f412765a2a5b603eb316360e6d8517e1e2

commit dc1ec1f412765a2a5b603eb316360e6d8517e1e2
Author: Jo-Philipp Wich <jo at mein.io>
AuthorDate: Tue Aug 2 15:29:40 2016 +0200

    Add link to signing on downloads page
    
    Signed-off-by: Jo-Philipp Wich <jo at mein.io>
---
 downloads.txt | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/downloads.txt b/downloads.txt
index f20993b..1400987 100644
--- a/downloads.txt
+++ b/downloads.txt
@@ -13,6 +13,9 @@ You can find the BuildBots activity in the following links:
 * Phase 1: http://phase1.builds.lede-project.org/builders[target/subtargets]
 * Phase 2: http://phase2.builds.lede-project.org/builders[packages]
 
+Please refer to link:signing.html[the file signing documentation] to learn how
+to verify the integrity of the firmware downloads.
+
 === Mirrors
 
 The contents of the download server are available on several mirrors as well.