Move Nginx reverse proxy sample from INSTALL to separate file

mods/sample-nginx-reverse-proxy.config

@@ -0,0 +1,30 @@
+#
+# Example of NGINX as reverse-proxy terminating an HTTPS connection.
+#
+# This is not a complete NGINX config.
+#
+# Please refer to NGINX docs
+#
+
+...
+
+server {
+
+	...
+
+	# assuming Friendica runs on port 8080
+	location / {
+		if ( $scheme != https ) {
+			# Force Redirect to HTTPS
+			return 302 https://$host$uri;
+		}
+		proxy_pass http://localhost:8080;
+		proxy_redirect off;
+		proxy_set_header Host $host;
+		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+		proxy_set_header Forwarded "for=$proxy_add_x_forwarded_for; proto=$scheme";
+	}
+
+	...
+
+}