docs: add explicit mention about RFC 6570

Author: char0n

README.md

@@ -254,6 +254,7 @@ The Server URL Templating is defined by the following [ABNF](https://tools.ietf.
 
 ```abnf
 ; OpenAPI Server URL templating ABNF syntax
+; Aligned with RFC 6570 (https://www.rfc-editor.org/rfc/rfc6570)
 server-url-template    = 1*( literals / server-variable )
 server-variable        = "{" server-variable-name "}"
 server-variable-name   = 1*( unreserved / pct-encoded / sub-delims / ":" / "@" )

src/server-url-templating.bnf

@@ -1,4 +1,5 @@
 ; OpenAPI Server URL templating ABNF syntax
+; Aligned with RFC 6570 (https://www.rfc-editor.org/rfc/rfc6570)
 server-url-template    = 1*( literals / server-variable )
 server-variable        = "{" server-variable-name "}"
 server-variable-name   = 1*( unreserved / pct-encoded / sub-delims / ":" / "@" )

src/server-url-templating.js

@@ -171,6 +171,7 @@ export default function grammar(){
   this.toString = function toString(){
     let str = "";
     str += "; OpenAPI Server URL templating ABNF syntax\n";
+    str += "; Aligned with RFC 6570 (https://www.rfc-editor.org/rfc/rfc6570)\n";
     str += "server-url-template    = 1*( literals / server-variable )\n";
     str += "server-variable        = \"{\" server-variable-name \"}\"\n";
     str += "server-variable-name   = 1*( unreserved / pct-encoded / sub-delims / \":\" / \"@\" )\n";