[Minor] Add doc and systemd unit file

README.md

+# encchecker
+
+## About
+
+Postfix-Submission Milter that detects illegal UTF-8 encoded headers (headers have to be 7bit only)
+
+Postfix has a built-in feature named "strict_7bit_headers", but it does not have a "warn/log-only" feature. If you want to
+investigate how many and which mails would be affected by "strict_7bit_headers = yes", you can run `encchecker` in "logonly" mode.
+
+## Features
+
+* Checks each header of an email for invalid 8bit characters
+* Can be run in "reject" or "logonly" mode
+
+# Table of contents
+
+1. [Install](#install)
+    * [Postfix integration](#postfix-integration)
+    * [Preparing a docker image](#preparing-a-docker-image)
+    * [Build from source](#build-from-source)
+    * [Server options](#server-options)
+
+# Install
+
+## Postfix integration
+
+The service is configured in Postfix like this...
+
+```
+smtpd_milters={ inet:127.0.0.1:4649, connect_timeout=10s, command_timeout=10s, content_timeout=10s, default_action=accept }
+```
+
+... if you use the docker-compose.yml file as provided.
+
+Back to [table of contents](#table-of-contents)
+
+## Preparing a docker image
+
+The simplest way to use the program is by using a docker image.
+
+```shell
+cd /path/to/Dockerfile
+docker build -t encchecker:latest .
+```
+
+Back to [table of contents](#table-of-contents)
+
+## Build from source
+
+### Requirements
+
+To build ratelimit from source, you need at least Golang version 1.16 or above.
+
+### Compile
+
+```shell
+cd /path/to/sourcecode
+go mod tidy
+go build -o encchecker .
+```
+
+You can find an example systemd unit file in the contrib folder.
+
+Back to [table of contents](#table-of-contents)
+
+## Server options
+
+```shell
+/usr/local/bin/encchecker --help
+```
+
+produces the following output:
+
+```
+Usage of /usr/local/bin/encchecker:
+  -addr string
+        Bind to address or unix domain socket (default "127.0.0.1:4649")
+  -logonly
+        Only log results without rejecting emails
+  -proto string
+        Protocol family (unix or tcp) (default "tcp")
+  -reply string
+        Custom response message (default "554 5.6.0 Message blocked due to illegal UTF-8 header encoding")
+  -verbose
+        Enable verbose output
+```
+
+Back to [table of contents](#table-of-contents)
+
+Hope you enjoy :-)
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contrib/encchecker.service

+[Unit]
+Description=Postfix Milter to check for broken UTF8-characters in header fields
+After=network.target
+
+[Service]
+Type=simple
+User=nobody
+Group=nogroup
+ExecStart=/usr/local/bin/encchecker -verbose -logonly
+Restart=on-failure
+
+[Install]
+WantedBy=multi-user.target
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