Add docs for commandline flags

docs/command-line-flags.md

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+# Command Line Flags
+
+Command line flags override the confd [configuration file](https://github.com/kelseyhightower/confd/wiki/Configuration-Guide), which is useful for testing and onetime runs. This also means that you can use confd without a configuration file.
+
+> Note: All flags use a single `-`, which is much different from GNU style flags that use `--`. So you have to use `-noop` and not `--noop`.
+
+To display the confd command line flags use the `-h` flag
+
+```Bash
+confd -h
+Usage of confd:
+  -client-cert="": the client cert
+  -client-key="": the client key
+  -confdir="/etc/confd": confd conf directory
+  -config-file="": the confd config file
+  -debug=false: enable debug logging
+  -etcd-scheme="http": the etcd URI scheme. (http or https)
+  -interval=600: etcd polling interval
+  -node=[]: list of etcd nodes
+  -noop=false: only show pending changes, don't sync configs.
+  -onetime=false: run once and exit
+  -prefix="/": etcd key path prefix
+  -quiet=false: enable quiet logging. Only error messages are printed.
+  -srv-domain="": the domain to query for the etcd SRV record, i.e. example.com
+  -verbose=false: enable verbose logging
+```