adds server url template abnf

src/oas.md

@@ -557,6 +557,23 @@ servers:
 
 An object representing a Server Variable for server URL template substitution.
 
+The server URL templating is defined by the following [ABNF](https://tools.ietf.org/html/rfc5234) syntax.
+
+```abnf
+server-url-template            = 1*( server-literal / template-expression )
+server-literal                 = 1*pchar
+template-expression            = "{" template-expression-param-name "}"
+template-expression-param-name = 1*( %x00-79 / %x7C / %x7E-10FFFF ) ; every UTF8 character except { and }
+
+pchar          = unreserved / pct-encoded / sub-delims / ":" / "@"
+unreserved     =  ALPHA / DIGIT / "-" / "." / "_" / "~"
+pct-encoded    =  "%" HEXDIG HEXDIG
+sub-delims     =  "!" / "$" / "&" / "'" / "(" / ")"
+               /  "*" / "+" / "," / ";" / "="
+```
+
+Here, `pchar`, `unreserved`, `pct-encoded` and `sub-delims` definitions are taken from [RFC 3986](https://tools.ietf.org/html/rfc3986).
+
 ##### Fixed Fields
 
 | Field Name | Type | Description |