Make 'processors' configurable as recommended in the guide

[elastic/elasticsearch-definitive-guide#679](elastic/elasticsearch-definitive-guide#679)

Author: jjungnickel

README.md

@@ -21,3 +21,5 @@ Besides the [inherited ones](https://github.com/pires/docker-elasticsearch#envir
 
 * [DISCOVERY_SERVICE](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-discovery-zen.html#unicast) - the service to be queried for through DNS (default = `elasticsearch-discovery`).
 * [MEMORY_LOCK](https://www.elastic.co/guide/en/elasticsearch/reference/current/important-settings.html#bootstrap.memory_lock) - memory locking control defaults to `false` as Kubernetes requires swap to be disabled.
+* [PROCESSORS](https://github.com/elastic/elasticsearch-definitive-guide/pull/679/files) - allow elasticsearch to optimize for the actual number of available cpus (must be an integer and can be derived from resources.limits - default = 1)
+

config/elasticsearch.yml

@@ -8,6 +8,8 @@ node:
   ingest: ${NODE_INGEST}
   max_local_storage_nodes: ${MAX_LOCAL_STORAGE_NODES}
 
+processors: ${PROCESSORS:1}
+
 network.host: ${NETWORK_HOST}
 
 path: