Declare ext-json as dependency

Author: PowerKiKi

README.md

@@ -43,7 +43,7 @@ This lib is an implementation of PSR-15, so it can be used with any
 framework supporting PSR-15. For specific configuration instructions, refer
 to your framework documentation.
 
-If your framework does not support PSR-15 middlewares, you will probably 
+If your framework does not support PSR-15 middleware, you will probably
 need some kind of bridge. Again, refer to your framework for specific instructions.
 Or else, you could use the direct usage below for manual integration.
 

composer.json

@@ -25,6 +25,7 @@
     },
     "require": {
         "php": "^7.1",
+        "ext-json": "*",
         "psr/http-server-middleware": "^1.0",
         "webonyx/graphql-php": "^0.13.0"
     },