Fix: Remove blockBindings from docs (fixes #307, fixes #339)

[Hirse]

This PR removes the mentioning of the blockBindings option from the Differences from Espree 2.x section in the README as it is no longer supported and the opt-in apparently not necessary anymore.

let and const declarations are no longer parsed by default. You must opt-in using ecmaFeatures.blockBindings.

[platinumazure]

Hi @Hirse, thanks for the pull request!

I believe one still has to opt in by specifying ecmaVersion: 2015 (or greater). ES5 is still the default, so let and const will not be parsed with the default settings.

[Hirse]

@platinumazure you are right. Based on some testing with the AST explorer there are two ways to enable support for let and const: Using ecmaVersion > 5 or setting sourceType to module. I will update the docs accordingly.