Add usage instructions

README.md

@@ -5,12 +5,43 @@
 [![Build Status](https://travis-ci.com/lukechilds/parcel-plugin-ogimage.svg?branch=master)](https://travis-ci.com/lukechilds/parcel-plugin-ogimage)
 [![npm](https://img.shields.io/npm/v/parcel-plugin-ogimage.svg)](https://www.npmjs.com/package/parcel-plugin-ogimage)
 
+Sets absolute URLs for `og:image` meta tags. This is required by the spec and relative URLs will not work on some sites such as Twitter.
+
+You can fix this directly in parcel by using `--public-url https://example.com`, however now all your URLs are hardcoded to absolute URLs which may be undesirable and can break things like prerendering.
+
+This plugin uses the value of the `og:url` meta tag to convert `og:image` to absolute URL.
+
 ## Install
 
 ```shell
 npm install parcel-plugin-ogimage
 ```
 
+## Usage
+
+Just install this package as a development dependency. Parcel will automatically call it when building your application.
+
+You **must** have both `og:image` and `og:url` meta tags:
+
+```html
+<meta property="og:image" content="card.png">
+<meta property="og:url" content="https://example.com">
+```
+
+Parcel will generate that into something like this:
+
+```html
+<meta property="og:image" content="/card.9190ce93.png">
+<meta property="og:url" content="https://example.com">
+```
+
+`parcel-plugin-ogimage` will then update the `og:image` with an absolute URL:
+
+```html
+<meta property="og:image" content="https://example.com/card.9190ce93.png">
+<meta property="og:url" content="https://example.com">
+```
+
 ## License
 
 MIT © Luke Childs