Make 'processors' configurable as recommended in the guide

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

Author: jjungnickel

README.md

@@ -122,6 +122,7 @@ This image can be configured by means of environment variables, that one can set
 * [SHARD_ALLOCATION_AWARENESS_ATTR](https://www.elastic.co/guide/en/elasticsearch/reference/current/allocation-awareness.html#CO287-1)
 * [MEMORY_LOCK](https://www.elastic.co/guide/en/elasticsearch/reference/current/important-settings.html#bootstrap.memory_lock) - memory locking control - enable to prevent swap (default = `true`) .
 * [REPO_LOCATIONS](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-snapshots.html#_shared_file_system_repository) - list of registered repository locations. For example `["/backup"]` (default = `[]`).
+* [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 - default = 1)
 
 ### Backup
 Mount a shared folder (for example via NFS) to `/backup` and make sure the `elasticsearch` user

config/elasticsearch.yml

@@ -7,6 +7,9 @@ node:
   data: ${NODE_DATA}
   ingest: ${NODE_INGEST}
   max_local_storage_nodes: ${MAX_LOCAL_STORAGE_NODES}
+
+processors: ${PROCESSORS:1}
+
 network.host: ${NETWORK_HOST}
 
 path: