Added documents for commandline arguments

docs/COMMAND_LINE_ARGS.md

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+Command Line Arguments
+========================
+# General
+
+You can see available commands with:
+```
+./mach -h
+./mach <sub-command> -h
+```
+Only arguments that need more explanation will be documented here.
+
+# Run
+## Enable Experimental Features
+Use `--pref` to enable experimental features like experimental DOM API, JavaScript API and CSS properties.
+
+e.g. To enable `flex` and `flex-direction` css properties:
+```
+./mach run -d -- --pref layout.flex.enabled --pref layout.flex-direction.enabled ...
+```
+
+You can find all the available preferences at [resources/prefs.json](http://mxr.mozilla.org/servo/source/resources/prefs.json).
+
+# Debugging
+## Remote Debugging
+Use `--devtools 6000` to start the devtools server on port 6000.
+
+e.g.
+```
+./mach run -d --devtools 6000 https://servo.org
+```
+
+To connect to the server, follow [this guide](https://developer.mozilla.org/en-US/docs/Tools/Remote_Debugging/Debugging_Firefox_Desktop#Connect).