Update installation instructions

This adds an @beta constraint for mongodb/mongodb until that library is stable.

README.md

@@ -30,7 +30,15 @@ The preferred method of installing this library is with
 [Composer](https://getcomposer.org/) by running the following from your project
 root:
 
-    $ composer require "alcaeus/mongo-php-adapter=dev-master"
+    $ composer require "alcaeus/mongo-php-adapter=dev-master" "mongodb/mongodb=@beta"
+
+If your project includes a library that requires `ext-mongo` you need to also
+specify a `provide` option in your composer.json:
+    "provide": {
+        "ext-mongo": "1.6.12"
+    }
+
+Due to a limitation in composer you need to specify this in the root package.
 
 # Known issues