readme: add a Requirements section, mentioning Go 1.8+

mxmCherry · mxmCherry · commit d4bd648cd857 · 2019-06-24T12:47:36.000+03:00
diff --git a/README.md b/README.md
@@ -4,6 +4,10 @@
 
 OpenRTB implementation for Go
 
+## Requirements
+
+Requires Go 1.8+ for proper `json.RawMessage` marshalling.
+
 ## Installation
 
 To install, use `go get`:
