Add note about partial ES9 support to readme
Add note about partial ES9 support to readme

Issue #569

ajaxorg/acorn,mysticatea/acorn,mathiasbynens/acorn,TimothyGu/acorn,ternjs/acorn,MarcelGerber/acorn,marijnh/acorn,ajaxorg/acorn,mathiasbynens/acorn,mathiasbynens/acorn,TimothyGu/acorn,MarcelGerber/acorn,marijnh/acorn,ternjs/acorn,mysticatea/acorn,MarcelGerber/acorn,ternjs/acorn,ajaxorg/acorn,mysticatea/acorn,marijnh/acorn,TimothyGu/acorn