Add "by version" per @jordansissel

This is a nice addition.  Thanks, @jordansissel

Fixes #3405

Author: untergeek

docs/asciidoc/static/repositories.asciidoc

@@ -5,9 +5,9 @@ We also have repositories available for APT and YUM based distributions. Note
 that we only provide binary packages, but no source packages, as the packages
 are created as part of the Logstash build.
 
-We have split the Logstash package repositories into separate urls to avoid 
-accidental upgrades across major or minor versions. For all 1.5.x releases use 
-1.5 as version number, for 1.4.x use 1.4, etc.
+We have split the Logstash package repositories by version into separate urls 
+to avoid accidental upgrades across major or minor versions. For all 1.5.x 
+releases use 1.5 as version number, for 1.4.x use 1.4, etc.
 
 We use the PGP key
 http://pgp.mit.edu/pks/lookup?op=vindex&search=0xD27D666CD88E42B4[D88E42B4],