need for php math functions. used in mergeFromJsonString (#3409)

composer.json

diff --git a/composer.json b/composer.json
index 5b6c7ee..80d90ea 100644
--- a/composer.json
+++ b/composer.json
@@ -11,6 +11,9 @@
   "require-dev": {
     "phpunit/phpunit": ">=4.8.0"
   },
+  "suggest": {
+    "ext-bcmath": "Need to support JSON deserialization"
+  },
   "autoload": {
     "psr-4": {
       "Google\\Protobuf\\Internal\\": "php/src/Google/Protobuf/Internal",