Document #1876 and --value-stdin.

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

Author: felixfontein

README.rst

@@ -1559,6 +1559,17 @@ 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:
+
+.. code:: 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]'
+
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