Add top N and bottom N filter functions, closes #241.

docs/sources/reference/functions.md

@@ -78,6 +78,38 @@ max(interval)
 ```
 **Deprecated**, use `aggregateBy(interval, max)` instead.
 
+
+Filter
+---------
+
+### top
+
+```
+top(N, value)
+```
+
+Returns top N series, sorted by _value_, which can be one of: _avg_, _min_, _max_, _median_.
+
+Examples:
+```
+top(10, avg)
+top(5, max)
+```
+
+### bottom
+
+```
+bottom(N, value)
+```
+
+Returns bottom N series, sorted by _value_, which can be one of: _avg_, _min_, _max_, _median_.
+
+Examples:
+```
+bottom(5, avg)
+```
+
+
 ## Trends
 
 ### trendValue