Updated README for cfengine up

Signed-off-by: Victor Moene <victor.moene@northern.tech>

Author: victormlg

README.md

@@ -99,12 +99,26 @@ groups:
       spawn: ubuntu
 ```
 
-up will spawn the necessary VMs and install cfengine using cf-remote
+`up` will spawn the necessary VMs and install cfengine using cf-remote
 
-```
+```bash
 cfengine up config.yaml
 ```
 
+To validate the config, one can run:
+
+```bash
+cfengine up --validate config.yaml
+```
+
+Every time `up` is run, it will do all the necessary (spawn, destroy, install, uninstall) to reconciliate the current state with the input configuration. After a successful run, the applied configuration is saved. On subsequent runs, CFEngine compares the previously applied configuration with the current configuration and applies only the necessary changes.
+
+To ignore the previously saved configuration and apply the current configuration as-is, run:
+
+```bash
+cfengine up --reset config.yaml
+```
+
 ## Supported platforms and versions
 
 This tool will only support a limited number of platforms, it is not intended to run everywhere CFEngine runs.