Add composer.json and update installation instructions

README.md

@@ -47,7 +47,31 @@ See a complete working example in the [roots-example-project.com repo](https://g
 
 ## Theme installation
 
-Clone the git repo - `git clone https://github.com/roots/sage.git` and then rename the directory to the name of your theme or website.
+Bottom line is you want to get the files in this repo into your local development environment. There are many ways to do this, two of which we will cover here.
+
+### via Command-line
+
+If you're already [using Composer to manage WordPress](https://roots.io/using-composer-with-wordpress/), then you might consider using composer's `create-project` command to download Sage.
+
+The example below assumes you're using Bedrock. If you're not, simply change the target path accordingly.
+
+```sh
+composer create-project roots/sage web/app/themes/your-theme-name-here
+```
+
+Then activate the theme via [wp-cli](http://wp-cli.org/commands/theme/activate/).
+
+```sh
+wp theme activate your-theme-name-here
+```
+
+### via WordPress Admin Panel
+
+1. [Download the latest release](https://github.com/roots/sage/releases/latest) of Sage.
+2. In your WordPress admin panel, navigate to Appearance->Themes
+3. Click Add New
+4. Click Upload Theme
+5. Upload the zip file that you downloaded.
 
 ## Theme setup