Add Italic support in README

[akamud]

A lot of people ask for Italic font support, which is something that I don't want to pack in the theme at the moment. This could be added as a customization on the user's VSCode and described on the README so people could use this extension and enable italic support if they wanted.

This looks like a good starting point to be added to the user VSCode theme:

{
"editor.tokenColorCustomizations": {
    "textMateRules": [
      {
        "scope": [
          "comment",
          "entity.name.type.class",
          "constant",
          "storage.modifier",
          "storage.type.class.js,storage.type.class.ts",
          "variable.language.this",
          "entity.other.attribute-name",
          "variable.parameter"
        ],
        "settings": {
          "fontStyle": "italic"
        }
      }
    ]
  },
  "editor.fontFamily": "Operator Mono Lig Light, monospace"
}

Explain on README that this can be tweaked to the user's liking.

Could be a workaround for #49, #27, and #61

[mblode]

Hey @akamud, thank you for this fantastic theme! I've released a fork of your theme that supports italics and has improved Markdown theming:

https://github.com/mblode/vscode-one-dark-italic