📚 Add n8n documentation page

docs/security.md

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+# Security
+
+By default, n8n can be accessed by everybody. This is OK if you have it only running
+locally but if you deploy it on a server which is accessible from the web you have
+to make sure that n8n is protected!
+Right now we have very basic protection via basic-auth in place. It can be activated
+by setting the following environment variables:
+
+```bash
+export N8N_BASIC_AUTH_ACTIVE=true
+export N8N_BASIC_AUTH_USER=<USER>
+export N8N_BASIC_AUTH_PASSWORD=<PASSWORD>
+```