📚 Add n8n documentation page

docs/setup.md

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+# Setup
+
+
+## Installation
+
+To install n8n globally:
+
+```bash
+npm install n8n -g
+```
+
+## Start
+
+After the installation n8n can be started by simply typing in:
+
+```bash
+n8n
+# or
+n8n start
+```
+
+
+## Start with tunnel
+
+!> **WARNING**: This is only meant for local development and testing. Should not be used in production!
+
+To be able to use webhooks which all triggers of external services like Github
+rely on n8n has to be reachable from the web. To make that easy n8n has a
+special tunnel service (uses this code: [https://github.com/localtunnel/localtunnel](https://github.com/localtunnel/localtunnel)) which redirects requests from our servers to your local
+n8n instance.
+
+To use it simply start n8n with `--tunnel`
+
+```bash
+n8n start --tunnel
+```