Add instructions to start threshold scan

Documenting the steps to run a threshold scan in P5

docs/operations/threshold-scan.md

+# Taking an s-bit threshold scan at P5
+
+## Starting the threshold scan
+
+To take a threshold scan, first connect to [RCMS](http://cmsrc-gem.cms:20000/rcms/gui/servlet/FMPilotServlet), then create a miniDAQ session or attach to a running session.
+
+From the RCMS interface, click on "Connect".
+
+Open the [xDAQ](http://srv-s2g18-33-01.cms:20100/urn:xdaq-application:service=calibration) calibration suite. In case the calibration suite page was already opened before you clicked on "Connect" in RCMS, reload the page manually.
+
+From the dropdown menu, select the calibration type "S-bit ARM DAC scan", then  set the number of iterations in the "Iterations" text box to 6 and leave all the other parameters to their default values. Click on "Apply settings".
+
+Go back to the RCMS interface, then follow these steps:
+1. click on "Configure" and wait for all the subsystems to be in _Configured_ state;
+2. click on "Start" and wait for the status of all subsystems to be _Running_;
+3. click on "Stop".
+
+Go back to the calibration suite in xDAQ, then click on "Run".
+
+While the scan is ongoing, from the P5 GEM DAQ machine check that the output files are being written to the subfolders `fed1467` and `fed1468` of the folder `/gemdata/runs/test/sBitRateScan/`.
+
+## Analyzing the threshold scan results