Add documentation for --password-command

changelog/unreleased/pull-2094

@@ -0,0 +1,8 @@
+Enhancement: Run command to get password
+
+We've added the `--password-command` option which allows specifying a command
+that restic runs every time the password for the repository is needed, so it
+can be integrated with a password manager or keyring. The option can also be
+set via the environment variable `$RESTIC_PASSWORD_COMMAND`.
+
+https://github.com/restic/restic/pull/2094

doc/030_preparing_a_new_repo.rst

@@ -21,6 +21,19 @@ using a local repository; the remaining sections of this chapter cover all the
 other options. You can skip to the next chapter once you've read the relevant
 section here.
 
+For automated backups, restic accepts the repository location in the
+environment variable ``RESTIC_REPOSITORY``. For the password, several options
+exist:
+
+ * Setting the environment variable ``RESTIC_PASSWORD``
+
+ * Specifying the path to a file with the password via the option
+   ``--password-file`` or the environment variable ``RESTIC_PASSWORD_FILE``
+
+ * Configuring a program to be called when the password is needed via the
+   option ``--password-command`` or the environment variable
+   ``RESTIC_PASSWORD_COMMAND``
+
 Local
 *****
 
@@ -41,11 +54,6 @@ command and enter the same password twice:
    Remembering your password is important! If you lose it, you won't be
    able to access data stored in the repository.
 
-For automated backups, restic accepts the repository location in the
-environment variable ``RESTIC_REPOSITORY``. The password can be read
-from a file (via the option ``--password-file`` or the environment variable
-``RESTIC_PASSWORD_FILE``) or the environment variable ``RESTIC_PASSWORD``.
-
 SFTP
 ****