Add basic daemon documentation

doc/adv-filt.md

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+# Advanced Filter
+
+## Postfix Filters
+
+There are two types of Postfix mail filters: Simple Filters and Advanced
+Filters.  Simple Filters are executed for each incoming email as a new
+process, which may turn out to be expensive in terms of resources.  Advanced
+Filters work as a mail-processing proxies.
+
+For detailed documentation, see [FILTER README](https://www.postfix.org/FILTER_README.html).
+
+## Installation
+
+Just use the following command to install dependencies:
+
+    pip install -r requirements.txt
+
+## Configuration
+
+Lacre Advanced Filter, also known as daemon, is configured in the `[daemon]`
+section of configuration file.  Two obligatory parameters to be defined there
+are:
+
+  * `host` -- IP address or a host name;
+  * `port` -- TCP port Lacre should listen on.
+
+The other very important section is `[relay]`, which by default uses Simple
+Filter destination.  It has to be adjusted for Advanced Filter to work,
+setting port to `10026`.
+
+Command to spawn a Lacre daemon process is:
+
+    GPG_MAILGATE_CONFIG=/etc/gpg-mailgate.conf PYTHONPATH=... python -m lacre.daemon
+
+Two environment variables used here are:
+
+  * `GPG_MAILGATE_CONFIG` (not mandatory) -- path to Lacre configuration,
+    unless it's kept in default location (`/etc/gpg-maillgate.conf`).
+  * `PYTHONPATH` (not mandatory) -- location of Lacre modules.  You can place
+    them below your Python's `site-packages` to be reachable by any other
+    Python software.