Update documentation

Author: dylanhitt

README.md

@@ -32,6 +32,7 @@ To configure the command a option function will be passed which receives the com
 
 Default option functions:
 
+ - `cmd.WithCustomBaseCommand(*exec.Cmd)`
  - `cmd.WithStandardStreams`
  - `cmd.WithCustomStdout(...io.Writers)`
  - `cmd.WithCustomStderr(...io.Writers)`

command.go

@@ -72,6 +72,17 @@ func NewCommand(cmd string, options ...func(*Command)) *Command {
 	return c
 }
 
+// WithCustomBaseCommand allows the OS specific generated baseCommand
+// to be overridden by an *os/exec.Cmd.
+//
+// Example:
+//
+//      c := cmd.NewCommand(
+//        "echo hello",
+//        cmd.WithCustomBaseCommand(exec.Command("/bin/bash", "-c")),
+//      )
+//      c.Execute()
+//
 func WithCustomBaseCommand(baseCommand *exec.Cmd) func(c *Command) {
 	return func(c *Command) {
 		baseCommand.Args = append(baseCommand.Args, c.Command)