Specify earliest supported Chrome version.

We support Chrome 55+.

Anbody using Chrome 23 or earlier would be affected by
#112

Anybody using old versions of Chrome is vulnerable to known
security issues, so we encourage people to use recent Chrome
versions.

Author: ericwilligers

docs/support.md

@@ -13,7 +13,7 @@ Browser support
 
 The polyfill is supported on modern versions of all major browsers, including:
 
-* Chrome
+* Chrome 55+
 * Firefox 27+
 * IE10+ (including Edge)
 * Safari (iOS) 7.1+