Revert "Removed provide configuration"

This reverts commit 54fa6119fc77941061c52ae5e3fedd73306f3756.

README.md

@@ -36,6 +36,14 @@ root:
 
     $ 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
 
 ## Mongo