add note about supporting dev-master of htmlpurifier

README.md

@@ -38,6 +38,22 @@ composer require p3k/xray
 
 You can also [download a release](https://github.com/aaronpk/XRay/releases) which is a zip file with all dependencies already installed.
 
+**Note** XRay uses the HTMLPurifier library, which is in the process of updating to support newer PHP versions. Until they release an update beyond `v4.9.3`, you will need to add the following to your `composer.json` file in order to tell your project it's okay to install the dev-master version:
+
+```json
+{
+  "require": {
+    "ezyang/htmlpurifier": "dev-master#6d6d88512a8146939a9161bb03e95e3e97840439@dev"
+  },
+  "repositories": [
+    {
+      "type": "git",
+      "url": "https://github.com/ezyang/htmlpurifier.git"
+    }
+  ]
+}
+```
+
 ### Parsing
 
 ```php
@@ -111,7 +127,7 @@ $mf2 = Mf2\parse($html, 'http://example.com/entry');
 $xray = new p3k\XRay();
 $parsed = $xray->process('http://example.com/entry', $mf2); // note the use of `process` not `parse`
 
-Array 
+Array
 (
     [data] => Array
         (