Update README

README.md

@@ -12,21 +12,21 @@ Command execution in Gulp.js.
 As opposed to other plugins or
 [`child_process.exec()`](https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback),
 `gulp-execa` uses the popular [`execa`](https://github.com/sindresorhus/execa)
-library:
+library providing:
 
 - [Better Windows support](https://github.com/IndigoUnited/node-cross-spawn#why),
-  including [shebangs](<https://en.wikipedia.org/wiki/Shebang_(Unix)>).
-- Faster and more secure, as it does not use a shell (by default).
-- Execution of locally installed binaries.
-- Descriptive errors and configurable verbosity.
-- Interleaved stdout/stderr.
+  including [shebangs](<https://en.wikipedia.org/wiki/Shebang_(Unix)>)
+- Faster and more secure commands, since no shell is used by default
+- Execution of locally installed binaries
+- Descriptive errors and configurable verbosity
+- Interleaved `stdout`/`stderr`
 
 Commands can be executed either directly or inside a files stream. In streaming
 mode, unlike similar libraries:
 
 - commands are run [in parallel](https://github.com/almost/through2-concurrent),
-  not [serially](https://github.com/rvagg/through2).
-- output can be saved either in files or in variables.
+  not [serially](https://github.com/rvagg/through2)
+- output can be saved either in files or in variables
 
 Example gulpfile:
 
@@ -68,5 +68,27 @@ Executes `command` and returns a promise.
 ## stream(function, [options])
 
 Returns a stream that executes a `command` on each input file. `function` must
-take the [file](https://gulpjs.com/docs/en/api/vinyl#instance-properties) as
+take a [file](https://gulpjs.com/docs/en/api/vinyl#instance-properties) as
 argument and return a `command`.
+
+# Command
+
+By default, no shell interpreter (like Bash or `cmd.exe`) is used. This means
+`command` must only be the program path followed by arguments. No
+escaping/quoting is needed, except for significant spaces (with a backslash).
+
+Shell features such as globbing, variables and operators (`&&`, `>`, `;`) should
+not be used. Fortunately, all of this can done directly in Node.js instead.
+
+Shell interpreters are slower, less secure and less cross-platform. However, you
+can still use them by specifying the `shell` option.
+
+```js
+const { series } = require('gulp')
+
+// Wrong
+module.exports.check = task('npm audit && npm outdated')
+
+// Correct
+module.exports.check = series(task('npm audit'), task('npm outdated'))
+```