Document getsops/sops#1876 and --value-stdin.

Converted from getsops/sops@d9be059

Signed-off-by: Felix Fontein <felix@fontein.de>

Author: felixfontein

content/en/docs/_index.md

@@ -1596,6 +1596,17 @@ The value must be formatted as json.
 $ sops set ~/git/svc/sops/example.yaml '["an_array"][1]' '{"uid1":null,"uid2":1000,"uid3":["bob"]}'
 ```
 
+You can also provide the value from a file or stdin:
+
+``` sh
+# Provide the value from a file
+$ echo '{"uid1":null,"uid2":1000,"uid3":["bob"]}' > /tmp/example-value
+$ sops set ~/git/svc/sops/example.yaml --value-file '["an_array"][1]' /tmp/example-value
+
+# Provide the value from stdin
+$ echo '{"uid1":null,"uid2":1000,"uid3":["bob"]}' | sops set ~/git/svc/sops/example.yaml --value-stdin '["an_array"][1]'
+```
+
 ## Unset a sub-part in a document tree
 
 Symmetrically, SOPS can unset a specific part of a YAML or JSON document, by providing