Add documentation

Signed-off-by: Tony Holdstock-Brown <tony@docker.com>

docs/configuration.md

@@ -78,6 +78,7 @@ information about each option that appears later in this page.
     storage:
       filesystem:
         rootdirectory: /var/lib/registry
+		maxthreads: 100
       azure:
         accountname: accountname
         accountkey: base64encodedaccountkey

docs/storage-drivers/filesystem.md

@@ -16,3 +16,7 @@ An implementation of the `storagedriver.StorageDriver` interface which uses the
 `rootdirectory`: (optional) The absolute path to a root directory tree in which
 to store all registry files. The registry stores all its data here so make sure
 there is adequate space available. Defaults to `/var/lib/registry`.
+`maxthreads`: (optional) The maximum number of simultaneous blocking filesystem
+operations permitted within the registry. Each operation spawns a new thread and
+may cause thread exhaustion issues if many are done in parallel. Defaults to
+`100`, and can be no lower than `25`.