Document the except option to mangle

Added documentation for the `except` option to the `mangle` option in the API reference.

README.md

@@ -676,6 +676,10 @@ Other options:
 - `parse` (default {}) — pass an object if you wish to specify some
   additional [parser options][parser]. (not all options available... see below)
 
+##### mangle
+
+ - `except` - pass an array of identifiers that should be excluded from mangling
+
 ##### mangleProperties options
 
  - `regex` — Pass a RegExp to only mangle certain names (maps to the `--mangle-regex` CLI arguments option)