Add CERTBOT_PLUGIN_PATH (#25)

* Add CERTBOT_PLUGIN_PATH

* Move to usage

* Change link text

* Move to troubleshooting

README.md

@@ -59,6 +59,14 @@ To acquire a certificate for ``*.example.com``
      -d '*.example.com'
 ```
      
+## Troubleshooting
+
+If `certbot plugins` does not show the installed plugin, you might need to set `CERTBOT_PLUGIN_PATH`.  
+```
+CERTBOT_PLUGIN_PATH=/usr/local/lib/python3.9/site-packages/ certbot renew
+```  
+[See letsencrypt community thread](https://community.letsencrypt.org/t/how-do-i-make-certbot-find-use-an-installed-plugin/198647/5)
+
 ## Thanks to
 
 Of course certbot, which examples and documentation I used to implement this plugin. And to https://github.com/m42e/certbot-dns-ispconfig which served as an excellent example and README template as well.