CLI docs (#121)

* Start of cli docs

* Document the CLI

Also adds support for the `--config` option and `--port` option for the dev server.

* Add tests for --config and --port flags

* Add port to validateConfig

1 files changed, 41 insertions, 0 deletions

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+## 👩🏽‍💻 Command Line Interface
+
+### Global Flags
+
+#### `--config path`
+
+Specify the path to the config file. Defaults to `astro.config.mjs`. Use this if you use a different name for your configuration file or have your config file in another folder.
+
+```shell
+astro --config config/astro.config.mjs dev
+```
+
+#### `--project-root path`
+
+Specify the path to the project root. If not specified the current working directory is assumed to be the root.
+
+The root is used for finding the Astro configuration file.
+
+```shell
+astro --project-root examples/snowpack dev
+```
+
+#### `--version`
+
+Print the Astro version number and exit.
+
+#### `--help`
+
+Print the help message and exit.
+
+### Commands
+
+#### `astro dev`
+
+Runs the Astro development server. This starts an HTTP server that responds to requests for pages stored in `astro/pages` (or which folder is specified in your [configuration](../README.md##%EF%B8%8F-configuration)).
+
+__Flags__
+
+##### `--port`
+
+Specifies should port to run on. Defaults to `3000`.
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