add readme

README.md

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+# rspamd
+
+## configuration
+
+### redis
+configure redis and tell rspamd to use it:
+```
+redis:
+  instances:
+    - name: rspamd
+      socket_enabled: true
+      maxmemory: 200M
+      maxmemory_policy: volatile-ttl
+    - name: rspamd-bayes
+      socket_enabled: true
+      maxmemory: 500M
+      maxmemory_policy: volatile-ttl
+    - name: rspamd-fuzzy
+      socket_enabled: true
+      maxmemory: 200M
+      maxmemory_policy: volatile-ttl
+
+rspamd:
+  local_configs:
+    redis.conf:
+      servers: /var/run/redis/redis-rspamd.sock
+    classifier-bayes.conf:
+      backend: redis
+      servers: /var/run/redis/redis-rspamd-bayes.sock
+    worker-fuzzy.inc:
+      backend: redis
+      servers: /var/run/redis/redis-rspamd-fuzzy.sock
+```
+
+### milter
+add rspamd to your smtpd_milters:
+```
+postfix:
+  smtpd_milters:
+    - "inet:localhost:11332"
+```
+
+## dkim signing
+
+rspamd can sign outgoing mails with dkim. the following steps are necessary:
+* create a keypair: `rspamadm dkim_keygen -d <domain> -s <selector>`
+  * domain should be the domain you want to sign mails for
+  * selector should be an unique identifier, for example: 2021020801
+* add dns record
+* create private key file at `/var/lib/rspamd/dkim/<domain>.<selector>.key`
+  * `chown _rspamd /var/lib/rspamd/dkim/*`
+  * `chmod u=r,go= /var/lib/rspamd/dkim/*`
+* create/update selector in `/etc/rspamd/dkim_selectors.map`: `<domain>   <selector>`