DM:tools: add README.rst for acrnlog tool

Signed-off-by: Yan, Like <like.yan@intel.com>
Reviewed-by: Zhao Yakui <yakui.zhao@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

devicemodel/tools/acrnlog/README.rst

@@ -0,0 +1,47 @@
+ACRNLOG
+#######
+
+DESCRIPTION
+###########
+acrnlog is a userland tool to capture ACRN hypervisor log, it runs as an
+SOS service at boot. It captures two kinds of logs:
+ - log of current running;
+ - log of last running if crashed and logs remaining.
+
+The path to save log files is /tmp/acrnog/, so the log files would be lost
+after reset.
+
+USAGE
+#####
+The acrnlog tool is launched as a service at boot, with 4 1MB log files limited.
+To change the log file limitation:
+
+- temporary change
+ Stop the acrnlog service:
+
+ # systemctl disable acrnlog
+
+ Restart acrnlog running at backgroud with size and number of files.
+ For example:
+
+ # acrnlog -n 8 -s 4 &
+
+Use get_loglevel/set_loglevel to query and change the hypervisor loglevel.
+The mem_loglevel controls log to be saved using acrnlog, while
+console_loglevel controls log to output to console. For example:
+ ACRN:\>get_loglevel
+ console_loglevel: 2, mem_loglevel: 4
+ ACRN:\>set_loglevel 2 5
+ ACRN:\>get_loglevel
+ console_loglevel: 2, mem_loglevel: 5
+
+- permanent chagne
+ Edit /usr/lib/systemd/system/acrnlog.service to attached the -n and -s options to
+ the ExecStart cmd, and restart the service. For example:
+
+ ExecStart=/usr/bin/acrnlog -n 8 -s 4
+
+BUILD&INSTALLATION
+##################
+ # make
+ copy acrnlog to /usr/bin/ and copy acrnlog.service to /usr/lib/systemd/system/