Add Tor instance installation doc

docs/onion.md

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+# Tor instance
+
+Install Tor.
+
+Install Mitra. Uncomment or add the following block to Mitra configuration file:
+
+```yaml
+federation:
+  proxy_url: 'socks5h://127.0.0.1:9050'
+```
+
+Where `127.0.0.1:9050` is the address and the port where Tor proxy is listening.
+
+Configure the onion service by adding these lines to `torrc` configuration file:
+
+```
+HiddenServiceDir /var/lib/tor/mitra/
+HiddenServicePort 80 127.0.0.1:8383
+```
+
+Where `8383` should correspond to `http_port` setting in Mitra configuration file.
+
+Restart the Tor service. Inside the `HiddenServiceDir` directory find the `hostname` file. This file contains the hostname of your onion service. Change the value of `instance_uri` parameter in Mitra configuration file to that hostname (it should end with `.onion`).
+
+Start Mitra.
+
+For more information about running onion services, visit https://community.torproject.org/onion-services/setup/