docs: Add some documentation and a README

docs/hooks.md

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+
+# Post-DATA hook
+
+After completion of DATA, but before accepting the mail for queueing, chasquid
+will run the command at `$config_dir/hooks/post-data`.
+
+The contents of the mail will be written to the command's stdin, and the
+environment is detailed below.
+
+If the exit status is 0, chasquid will move forward processing the command,
+and its stdout should contain headers which will be added to contents of
+the email (at the top).
+
+Otherwise, chasquid will respond with an error, and the last line of stdout
+will be passed back to the client as the error message.
+If the exit status is 20 the error code will be permanent, otherwise it will
+be temporary.
+
+
+This hook can be used to block based on contents, for example to check for
+spam or virus.  See `etc/hooks/post-data` for an example.
+
+
+## Environment
+
+This hook will run as the chasquid user, so be careful about permissions and
+privileges.
+
+The environment will contain the following variables:
+
+ - USER
+ - SHELL
+ - PATH
+ - PWD
+ - REMOTE_ADDR
+ - MAIL_FROM
+ - RCPT_TO (space separated)
+ - AUTH_AS (empty if not completed AUTH)
+ - ON_TLS (0 if not, 1 if yes)
+ - FROM_LOCAL_DOMAIN (0 if not, 1 if yes)
+ - SPF_PASS (0 if not, 1 if yes)
+
+There is a 1 minute timeout for hook execution.
+It will be run at the config directory.
+