Add description of forwards-compatible behavior.

Author: ehuss

src/doc/rustc/src/symbol-mangling/v0.md

@@ -25,6 +25,14 @@ There is no standardized demangled form of the symbols,
 though suggestions are provided for how to demangle a symbol.
 Implementers may choose to demangle in different ways.
 
+## Extensions
+
+This format may be extended in the future to add new tags as Rust is extended with new language items.
+To be forward compatible, demanglers should gracefully handle symbols that have encodings where it encounters a tag character not described in this document.
+For example, they may fall back to displaying the mangled symbol.
+The format may be extended anywhere there is a tag character, such as the [type] rule.
+The meaning of existing tags and encodings will not be changed.
+
 ## Grammar notation
 
 The format of an encoded symbol is illustrated as a context free grammar in an extended BNF-like syntax.