[DOCS] Adds CA Fingerprint docs

Author: picandocodigo

docs/connecting.asciidoc

@@ -124,6 +124,23 @@ Elasticsearch::Client.new(
 )
 ------------------------------------
 
+[discrete]
+[[ca-fingerprint]]
+==== CA fingerprint
+
+You can configure the client to only trust certificates that are signed by a specific CA certificate (CA certificate pinning) by providing a `ca_fingerprint` option. This will verify that the fingerprint of the CA certificate that has signed the certificate of the server matches the supplied value:
+
+[source,ruby]
+------------------------------------
+ca_fingerprint = '64F2593F...'
+client = Elasticsearch::Client.new(
+  host: 'https://elastic:changeme@localhost:9200',
+  transport_options: { ssl: { verify: false } },
+  ca_fingerprint: ca_fingerprint
+)
+------------------------------------
+
+The verification will be run once per connection.
 
 [discrete]
 [[client-usage]]