Update minimum Hugo version in README

[apraditya]

I'm trying to make this wonderful theme as my base for my site revamp and I was using Hugo v0.64.1. I am not aware of the current Hugo version and its features so I was surprised with the following error when I'm running the example site hugo server --themesDir ../.. on the exampleSite folder:

Building sites … ERROR 2020/07/31 20:30:10 render of "page" failed:
"/Users/apraditya/labs/hugo/devmuslimid/themes/zen/layouts/_default/baseof.html:2:65":
execute of template failed: template: _default/single.html:2:65: executing
"_default/single.html" at <.Site.Language.LanguageDirection>: can't evaluate field
LanguageDirection in type *langs.Language

Looks fine on the template: _default/single.html:2:65. After some time, I found out that my Hugo version was older than this theme required. So I thought I'd update this in the README.

[frjo]

Thanks! It is mentioned in the Multilingual section of the README but better to set it as an minimum version,