borg create: document how to backup stdin, fixes #2013

borg/archiver.py

@@ -1311,8 +1311,12 @@ class Archiver:
 
         create_epilog = textwrap.dedent("""
         This command creates a backup archive containing all files found while recursively
-        traversing all paths specified. The archive will consume almost no disk space for
-        files or parts of files that have already been stored in other archives.
+        traversing all paths specified. When giving '-' as path, borg will read data
+        from standard input and create a file 'stdin' in the created archive from that
+        data.
+
+        The archive will consume almost no disk space for files or parts of files that
+        have already been stored in other archives.
 
         The archive name needs to be unique. It must not end in '.checkpoint' or
         '.checkpoint.N' (with N being a number), because these names are used for