Add documentation about multi-instance mode

docs/MULTIPE_INSTANCES.md

@@ -0,0 +1,23 @@
+### We could run multiple instances of FastNetMon on same server
+
+First of all please create multiple configuration files for each instance.
+
+In each file you should specify uniq paths for following options for each separate instance:
+```bash
+# Path to pid file for checking process liveness 
+pid_path = /var/run/fastnetmon.pid
+
+# Path to file where we store information for fastnetmon_client
+cli_stats_file_path = /tmp/fastnetmon.dat
+```
+
+Next you need to specify custom path to configuration file:
+```bash
+./fastnetmon --configuration_file=/etc/fastnetmon_mayflower.conf --daemonize
+./fastnetmon --configuration_file=/etc/fastnetmon_hetzner.conf --daemonize
+```
+
+FastNetMon cli client expects stats file with default path /tmp/fastnetmon.dat so you need to set custom path with environment variable:
+```bash
+cli_stats_file_path=/tmp/fastnetmon_second_instance.dat  ./fastnetmon_client
+```