Add normal worker documentation

doc/markdown/workers/normal.md

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+# Rspamd normal worker
+
+Rspamd normal worker is intended to scan messages for spam. It has the following configuration options available:
+
+* `mime`: turn to `off` if you want to scan non-mime messages (e.g. forum comments or SMS), default: `on`
+* `allow_learn`: turn to `on` if you want to learn messages using this worker (usually you should use [controller](controller.md) worker), default: `off`
+* `timeout`: input/output timeout, default: `1min`
+* `task_timeout`: maximum time to process a single task, default: `8s`
+* `max_tasks`: maximum count of tasks processes simultaneously, default: `0` - no limit
+* `keypair`: encryption keypair
+
+## Encryption support
+
+To generate a keypair for the scanner you could use:
+
+    rspamadm keypair -u
+
+After that keypair should appear as following:
+
+~~~nginx
+keypair {
+    pubkey = "tm8zjw3ougwj1qjpyweugqhuyg4576ctg6p7mbrhma6ytjewp4ry";
+    privkey = "ykkrfqbyk34i1ewdmn81ttcco1eaxoqgih38duib1e7b89h9xn3y";
+}
+~~~
+
+You can use its **public** part thereafter when scanning messages as following:
+
+    rspamc --key tm8zjw3ougwj1qjpyweugqhuyg4576ctg6p7mbrhma6ytjewp4ry <file>