Fix #234 -- document messages required by sidebar plugin

Signed-off-by: Chris Warrick <kwpolska@gmail.com>

v7/sidebar/README.md

@@ -3,3 +3,10 @@ This plugin renders a sidebar `output/sidebar-LANG.inc` defined by a template `s
 The generated include file, one per language, must be somehow included in the rest of the blog. This can be done by using JavaScript to dynamically include the file, or by using tools like [File Tree Subs](https://github.com/felixfontein/filetreesubs/).
 
 The maximum number of most recent posts can be changed by defining `SIDEBAR_MAXIMUM_POST_COUNT = number` in the Nikola configuration.
+
+Note that the default templates expect some extra messages (translations), namely:
+
+* Recent Posts
+* Archives
+* Categories
+* Tags