add ruler sharding guide and update changelog

Signed-off-by: Jacob Lisi <[email protected]>

Author: jtlisi

CHANGELOG.md

@@ -3,7 +3,8 @@
 ## master / unreleased
 
 * [ENHANCEMENT] metric `cortex_ingester_flush_reasons` gets a new `reason` value: `Spread`, when `-ingester.spread-flushes` option is enabled.
-
+* [CHANGE] Remove unnecessary configs/flags from the ruler ring config to align with the pattern used in the distributor ring. 
+    * Ruler ring related flags are now all prefixed with `ruler.ring.` as opposed to just `ruler.`
 * [CHANGE] Flags changed with transition to upstream Prometheus rules manager:
   * `ruler.client-timeout` is now `ruler.configs.client-timeout` in order to match `ruler.configs.url`
   * `ruler.group-timeout`has been removed

docs/guides/sharded_ruler.md

@@ -0,0 +1,26 @@
+---
+title: "Config for horizontally scaling the Ruler"
+linkTitle: "Config for horizontally scaling the Ruler"
+weight: 4
+slug: ruler-sharding
+---
+
+## Context
+
+One option to scale the ruler is by scaling it horizontally. However, with multiple ruler instances running they will need to coordinate to determine which instance will evaluate which rule. Similar to the ingesters, the rulers establish a hash ring to divide up the responsibilities of evaluating rules. 
+
+## Config
+
+In order to enable sharding in the ruler the following flag needs to be set:
+
+```
+  -ruler.enable-sharding=true
+```
+
+In addition the ruler requires it's own ring to be configured, for instance:
+
+```
+  -ruler.ring.consul.hostname=consul.dev.svc.cluster.local:8500
+```
+
+The only configuration that is required is to enable sharding and configure a key value store. From there the rulers will shard and handle the division of rules automatically.